For people writing more involved code onto GHC than the normal API (GHC.hs) allows, this is a very useful function. In previous versions of GHC, it has always been unclear to me how I should run the type checker on, for example, a single expression, while keeping the context (i.e. when running the type checker again, the types derived in the former run should still be known in the latter). Simply having runTcInteractive exported does wonders for self-documentation purposes.

(patch to follow shortly)