While it may be unwise to tell others that they're just believing in belief, it may well be a good thing for rationalists to check on about themselves. I think believing in belief doesn't just appear in religious matters, but also in a belief that it's necessary to feel sure of one's beliefs in order to express them forcefully enough for other people to notice.

Questioning other people's motives is sufficiently insulting that I suggest doing it quite cautiously if you're actually trying to change people's minds rather than doing a superiority dance.

However, there are people who promote belief in belief explicitly-- for example, saying that it's a very bad thing to shake other people's religious faith, and I don't see a problem with arguing against that sort of thing.