I've been learning how to use FontForge and am now getting my feet wet with kerning. I see that there are two options to do kerning: classes and pairs.
To me, classes seem to have no downside. When I'm setting a 'T' to a 'c', 'e', and 'o', I can do them all at once! If I have something that's a bit of an oddity, like an 'a' or an 's', I can just set it by itself (knowing too that once I get into diacritics those characters won't be alone anymore).
So, why would I use pairs at all? Is it just a legacy thing since pairs came before classes? Does setting up a class introduce a lot of overhead into the font file that can bloat it or slow it down?