It CAN be used it's just not really smart, as ships might end up colliding with planets or the like. Traveling at warp speeds would move a ship so fast that it would be extremely hard to time the moment to drop out of warp.
It can. The Klingon Bird of Prey "H.M.S. Bounty" even engages its warp drive while in the Earth's atmosphere over the Bering Sea or the Gulf of Alaska during Star Trek IV: The Voyage Home. ProfessorTrek 03:03, July 1, 2011 (UTC)