I don't think the angled toptube was intended to affect wheelbase. In mountain bikes, it helps decrease standover height. In road bikes, Giant was the first to popularize angled toptubes in their "compact" frames. I think that was in the mid to late 90s. I've never really figured out why riders thought this was better than a level toptube on a road bike. I'm sure Giant used it to claim a lighter frame since they were essentially transferring metal from the frame to the seat post. Lately, angled toptubes have been a way to increase headtube length without increasing standover. That way, with threadless stems, the bars can come up higher without a huge stack of spacers or a really high rise stem. That's my impression anyway. I still like a level toptube.