As the one who added the religion Item, i must admit i am biased towards saying Yes. But that doesn't means i can't back up my answer with arguments and reasons. It just means that these arguments might be rationalizations which i haven't yet seen as such, so please take everything said here with a grain of salt.
I think we should accept religion items and rankings because they are an important part of society, because there are little to none other places that rank them, and because it can spark important and meaningful discussion.
First, society. Religions have risen and fallen throughout history, shaping it and the societies and their ethos. The fact that such an important part of, not just our lives, but the space and time around us is left outside of "common sense" judgement is nothing less than worrisome, and their impact on the world at large deserves to be, if nothing else, rated and scrutinized.
Second, the need to rank them. How can you rank something so big and influential out of 10, when there are so many points of view and clever arguments to analyze? I say we begin with an easy, biased and carefree approach. If your life has been impacted positively by Christianity, maybe it deserves an 8 or a 9? If you just can't let go the influence of Christianity on the Dark Ages, and think it should burn in the enlightenment stake along with all the other ideas that are holding humanity back, maybe it should get a 2 or a 1? And if, after following that line of reasoning to its natural conclusion, you feel like other people have made important arguments for or against the item at hand which counter your own view of it, maybe said rating could be adjusted? I agree that ranking Buddhism or Calvinism as you'd rate your favorite coffee brand feels weird and counter-intuitive, but that can be adjusted as time passes.
Finally, the discussion. As stated below, "Religious discussion is fine". I think this is something we can all agree on, but i'd rather not have us just stop there. To discuss something is a purely intellectual act, which can influence people who take the time to read and understand what's being said about the topic. But not everybody belongs to the bubble of academia. Most users won't really care enough about the religious discussion to step into the fray and spend their time learning about obscure excerpts from some holy book that are being used as arguments against common sense and logic. I believe here's where numbers are useful. To rate something out of 10 implies giving a personal opinion, one that differs from any objective statement brought up or conceded, one that, at its essence, is what this website is built for. Discussion brings up good points and bad points, and might change a few minds or allow certain persons to show off their knowledge of history, religion or philosophy, but to rate an item, specially an item as controversial as a religion, is to give a pure, unadultered opinion that often rises up from the deepest part of your self.