They do have multiple offworld bases that some missions are launched from- not quite a colony in the tens of thousands, but it's a start for what they need it for. And, at this point, they're not quite as safe as using Earth is- there's a weapons platform built by the Ancients discovered in Antarctica that lends some degree of protection. Combined with our satilite coverage, military forces and, as a last resort, the general public, it makes it more difficult for enemies to make an incursion onto the planet without us knowing about it. The producers have, however, expressed interest in the notion of using the concept of full disclosure of the Stargate to the public as the basis for an eventual theatrical film and launching point for a third series. At this point so far though, it just would probably do more harm than good, and distract us at home with the political, economical and socialogical ramifications when we're trying to hold back the Ori.