I'm keeping them, hearthfire will probably require you to use them again.
DLCs can not interact with another. (Fallout NV is a good example: Weapons from the Guns Runners DLC can not use perks of other DLCs)
Imagen that some people will buy hearthfire but do not own dawnguard.
Heathfire try to load content from the dawnguard DLC -> loading error -> crash or broken quest.
Edit:
The dragon scroll is stuck in my inventory and can not be sold to the college. (I finished MQ and sold it once bevor i started Dawnguard.)