Well, I think it’s a question of manners as much as anything. If you don’t wear one you risk being under-dressed, a much worse faux pas and potentially an insult to the interviewers than being overdressed.
Not wearing a suit shows that you are only prepared to maintain the minimum standard you can get away with.
Wearing a suit when it is maybe (but you don’t know for sure) not required shows that you like to present yourself well and maintain a high standard, and that you actually own a suit.
And anyways, is there an HR bod at the interview ? You can generally rely on them to be suited up.