That's true, but "rationalist" is used extremely frequently as a noun or adjective. (Google claims about four thousand hits on LW for the plural "rationalists.") The word "rationality" indeed has a meaning separate from the traditional polemical sense of "rationalism," and it's not too far from what's commonly meant by it on LW. However, "rationalist" is not separable from "rationalism."
However, "rationalist" is not separable from "rationalism".
Word forms are very important when we are talking about connotations. "Rationalist" is a different story, see current version of the post.
(Added google results data on "rationalist" to the post.)
I feel that the term "rationalism", as opposed to "rationality", or "study of rationality", has undesirable connotations. My concerns are presented well by Eric Drexler in the article For Darwin’s sake, reject "Darwin-ism" (and other pernicious terms):
So, my suggestion is to use "rationality" consistently and to avoid using "rationalism". Via similarity to "scientist" and "physicist", "rationalist" doesn't seem to have the same problem. Discuss.
(Typical usage on Less Wrong is this way already, 3720 Google results for "rationality" and 1210 for "rationalist", against 251 for "rationalism". I've made this post as a reference for when someone uses "rationalism".)