They would likely use the reaction to the Halo trilogy being released to decide whether or not to bring over ODST, Reach, and eventually 4, and possibly release future installments on PC alongside the next-gen console versions.
Do they really need to release those Halo on Steam to know that Halo 4 would sell gangbusters on PC ?
That's obvious.
But instead of giving us Gears 2/3 or Halo 4 they opt for Halo 3.