EDIT: Lumifer says:
"...when you are talking to someone, presumably you want to communicate some meaning and saying "I'm a rationalist" communicates very little."
This is true. Belief-labels are useful socially; as long as you don't come to internalize them, you should be fine. I would dissuade anyone from actually identifying with a belief-label, but YMMV.
George is a Perfect Bayesian Rationalist, and has recently come to the conclusion that everything Albert Camus says is correct with a probability of greater than 0.99999. Since his realization, George has called himself an absurdist. Why shouldn’t he, after all? He agrees with all that the absurdists say.
I will henceforth refer to words such as “absurdism”, “nihilism”, and “theism” as belief-labels, for obvious reasons. (Hopefully, this extensional definition should suffice.) I will discuss why I am more-or-less opposed to their usage, and why I don’t think rationalists should use them to describe themselves.
In George’s universe, absurdism is Perfectly Rational (as an example; let’s not debate absurdism in the comments). Because of this, if somebody is a (perfect) rationalist, then they agree with the tenants of absurdism. However, if someone is an absurdist, then they are not necessarily a rationalist – they may have adopted absurdism because it appealed to them, or because of social pressure, etc. Therefore, if George must choose between just describing himself as a rationalist, or just describing himself as an absurdist, he would do better to choose the former; it communicates more information, as the relationship between rationalism and absurdism is (in our example) that of implication, not equivalence. “I am a rationalist” implies “I believe the tenets of absurdism”; so does “I am an absurdist”, but the latter does not imply “I am a rationalist,” which the former obviously does.
You might say, “it is a false dichotomy.” And you would be right to do so, as George can, without expending significantly more effort, say: “I am a rationalist and an absurdist.” However, why would he say such a redundant thing? If one implies the other, then the other can be dropped.
Now, from the perspective of those to whom George is introducing himself, it might be the case that he is an imperfect rationalist, and therefore is not an absurdist. This is one argument for using the belief-label in this particular example; to better illustrate my next point, I shall choose a particularly salient example.
Consider the belief-label “atheism”. Rationalists of varying levels almost universally reject the tenets of theism, so when somebody says “I am a rationalist,” you can be almost sure that they are an atheist. And this is why I do not call myself an atheist: it would be misleading to call myself an atheist, but not a rationalist; and it would be redundant to call myself an atheist and a rationalist.
There is another problem with belief-labels, however, that transcends any semantic considerations. When you use a belief-label, you are separating the belief from the reasons you believe it. Yes, that is important enough to warrant italicization, bolding, underlining, and size-16 text. Why? Because it is a Deadly Mistake; a misstep in the dance, to borrow from Eliezer.
Beliefs are connected. It just doesn’t do to extrapolate a new belief from evidence, then say, “better add that to my list of beliefs!” If you let any part of your map float free, apart from the rest, then, upon encountering something that undermines a should-be-connected part, you may fail to properly update the floating part. If you keep your beliefs separated, in different mental boxes, then you make it easier for yourself to double-count previous evidence when you encounter new evidence. Atheism is not its own thing; it is inextricably linked to the rest of your beliefs. If it is an inevitable consequence of your rationalism, why give it a special name? Every piece of your map has its place, and should not forget it. If you cut up a real map, and glue it back together another way, then will it still represent the territory?
“Okay,” you say, “but cannot I call myself an atheist, without forgetting where my atheism belongs relative to my other beliefs?” Yes, you can. However, I do not recommend you do so, as it is very risky. Do not underestimate your susceptibility to bias; the longer you go on calling yourself an atheist, or an absurdist, or even a singularitarian, the more you risk forming an attachment to that belief. When the so-called New Atheists refer to themselves (rather proudly) as “atheists”, it almost implies that they have an attachment to the particular belief of atheism, and makes me imagine them defending it in every case, even those ones in which the theists are correct. If Richard Dawkins encountered strong evidence for theism, would he be able to overcome his cognitive dissonance correctly, and update his beliefs in the right direction? I do not know; however, I suspect it would be easier for him to do so if he identified as a rationalist, rather than an atheist. The virtue of a belief does not lie in the belief itself, but rather is a function of and only of the belief’s rationality. When you adopt a belief-label as part of your identity, you risk seeing the belief as its own separate thing, in its own mental box.
And before you talk about social benefits...
“I’m a republican.” “I’m a fan of Lost.” “I’m a transhuamist.” What do these statements have in common? They do not all include belief-labels, but they are all ways of cheering for a team, of belonging to a tribe. That is a glaring alarm-bell; TRIBALISM AND RATIONALITY DO NOT MIX. One who says “I am a socialist,” risks bias towards believing future statements made by socialists; furthermore, they make it harder for themselves to update their beliefs in a direction that they think conflicts with socialism.
Why not just call yourself a rationalist?