Isn't a Vampire Lord with a whole arsenal of spells, bat dodge, vampire vision, hovering, and etc. enough? Making crossbows Dawnguard exclusive will make the Dawnguard more appealing. Then again, we haven't seen much of what the Dawngaurd can do so far.
It's not about that. My thief character is a vampire and I'm joining them simply because he is a vampire and wouldn't hunt his own kind. I've always envisioned my thief character to use a crossbow and it seems silly to lock a brand new weapon type to one faction and take it away from players who choose the other faction, especially considering crossbows was a big thing
alot of people requested. Plus, TES has always been about be who you want to be, and I don't think it'd be that gamebreaking to be a crossbow using vampire.
I'm going Dawnguard on my first playthrough on my Nord character but I really hope I can use crossbows on characters that decide to roll vamp. Surely Dawnguard can get other things to balance the choice out. Besides, it's also an RP factor, some people don't want to be vampire characters or aren't an evil archetype which is incentive enough to side with Dawnguard.