I'm a backpack commuter anyway, so my choice of bike doesn't have any consequences in that respect. Even my commuter bike isn't 100% dedicated to commuting - it's my backup race bike during cyclocross season. So I'm accustomed to switching it up between 2-3 bikes, and I really like riding my road bike. I have a 7-8 mile commute, depending on route, and a fair amount of climbing no matter which way I go. So I find it much more pleasant on a road bike than anything else. I did just plot out a route that should considerably reduce the climbing I need to do, so I'm going to check it out, and who knows.
Also, I'll just note that while I prefer riding my road bike, speed doesn't have much to do with it. My current typical route has a lot of crossings and lights, so it's not very fast no matter I ride. But the climbing is much more pleasant on a road bike, and it's just more fun on a road bike.