In effect, they (though fully well-meaning) create their own derech. I submit that the “derech” of choosing one’s own derech is faulty. Especially if one wishes to follow the path of Chassidus, one should know that at the core of Chassidus lie the relationship of Rebbe and chossid, and the concept that only a true Tzaddik and Rebbe can create a derech in avodas Hashem.
As explained here, the Rebbeim pave the way for us through their example (there it is said in the context of the Chabad Rebbeim, but I would think that it applies to the Rebbeim of other groups too; in any case, this post is not about the specific idea of a Chabad Rebbe). The reason that they do this and that we do not, is that they are pure and holy Tzaddikim who are vastly superior and in a different league from us.
However, when the person hasn’t submitted to any one derech, he must “pick and choose.” He doesn’t do everything, because that would be overwhelming and impossible. So he selects different practices and approaches from different sources. What he fails to grasp is that such spiritual things are not given for regular people to understand with direct knowledge and thus make their own assessments about. They are not worldly matters about which every Beryl and Shmeryl is equally entitled to an opinion, as they are, let’s say, when it comes to which political party to vote for.
As the Rebbe puts it (Igros Kodesh, Vol. 3, p. 412):
Why should we go around making our own calculations, and calculate ourselves what is better for us spiritually—what do we know about spirituality altogether? If only we would know its existence [“metzius”]; we certainly don’t know its essence [“mehus”]. [How can we make such calculations of our own, when] we have been issued a directive [on the matter] by the Nasi HaDor [Leader of the Generation]. The Nasi is the heart of the Jewish people, and the level of heart is higher than the level of mouth.As far as the present discussion is concerned, we see from the above that someone who thinks that he can make assessments about spiritual matters on his own is foolish. What does he know about spiritual matters? Spiritual realities [“mehus”] are completely beyond his powers of perception. Even with regard to their external effects [“metzius”], he lacks understanding. Thus, the humble person will recognize that he should follow a Rebbe, who is a far superior kind of person (for more on the topic of Tzaddik and Rebbe in general, see here), and one who sees the reality of G–dliness (see here). From this incomparably superior vantage point, the Rebbe can truly guide the person to connect with Hashem.
Moreover, as explained in Chassidus, the Nasi HaDor is the “intermediary who joins” [“memutza hamechabeir”—for further explanation, see here] us with the very Essence of the Infinite One, may He be blessed. So if he directed one to a place of spiritual livelihood, this is surely the best thing possible for the Neshama and for the health of the body. And the Neshama will be healthy when it is a vessel to carry out G–d’s will.
Even someone who is without doubt significantly more inspired, intelligent, and spiritually sensitive than an average person is simply not qualified to formulate his own derech. He is not capable of taking into consideration all the sublime calculations that Tzaddikim and Rebbeim can through their superior perception and wisdom, and through their ruach hakodesh. A non-Rebbe will most likely err, whether on the side of being overly lenient, or being overly stringent. (This is all aside from the fact that every person has biases that cloud his judgement, which necessitates that heu consult with an objective outsider.)
Along these lines, the Talmud states (Eruvin 6b): “The halachah is like Beis Hillel. If one adopts the stringencies of both Beis Shammai and Beis Hillel, he is a fool who walks in darkness; if he adopts the leniencies of both, he is a rasha (wicked one).” I am not saying that someone who does not have a Rebbe and a specific derech is necessarily guilty of the above, but he is definitely more susceptible to doing so.
Moreover, what will the person do if he learns that there’s an inherent conflict between some of the elements within the two (or more) paths? Such a person will choose the one that makes more sense to him, and/or with which he feels more comfortable. But what if he doesn’t even notice that conflict? In addition to all that was said above about the fallacy of making such a mishmash, understand that some approaches and practices are just not meant to be combined, and when they are, the results are not favorable.
By way of analogy, a cook knows that certain foods taste well when mixed together in the correct proportion, but others clash and may even create a dish that is inedible, despite the tastiness of each individual ingredient. Another analogy, carrying on from the analogy of a path to a particular destination, is that if a person tries to travel down two different roads simultaneously, he will not reach his destination. Yet another analogy is from medicines. Chassidus is described (see this post) as the medicine for the exile. Perhaps it can be extrapolated from this that the derech that each Rebbe prescribes is akin to a specific prescription of medicine, and mixing medicines is not safe, as is obvious.
Finally, the Rebbe-chossid relationship, in which the chossid obeys the Rebbe as his master (“adoneinu”) even if he doesn’t understand, keeps the chossid in check. The Radziner Rebbe, Reb Gershon Chanoch Henech Leiner, OBM, once commented when he saw someone who declared himself a chossid without a Rebbe: “Git a kuk oif a hunt ohn a baal haboss!”—“Look at a dog without an owner!” (I heard this story in the name of Rabbi Groner of Melbourne, alav hasholom, related by his son-in-law.) Likewise, chassidim have an animal soul, which may even at times be akin to a dog. By following a Rebbe, the chossid keeps his animal soul in check and inculcates in himself a sense of subservience so that his animal soul is appropriately sublimated. But when the dog is his own master ...
In summary, a chossid should choose a specific Rebbe and derech in avodas Hashem because these are fundamental to the derech of Chassidus. One who fails to do so, and picks and chooses, in effect arrogates to himself authority to which he is not entitled, because only a Rebbe, by virtue of his lofty spiritual level, is capable of creating a true derech. The person may choose too many stringent or too many lenient ideas and practices, and different elements selected from various derachim may clash. However, when the chossid has the humility to follow a Rebbe and that Rebbe’s derech, doing so subdues the chossid’s animal soul and enables him to sublimate it.