Genuinely curious - Why? We were considering doing so at one point for PtP links (equiv to a /30 or a /31 if you area really bleeding edge) - but then decided that there wasn't any reason to not just use /64s everywhere.

I take your point though - it certainly is theoretically (and in practice) possible to doing something other than a /64 - I've just never seen it in the wild.

I have heard of one good reason: with a /64, someone sending packets to random addresses within that /64 will fill up the neighbor cache, while with a smaller network like a /126, that can't happen.