Jack Daniel
Legend
So, for me, "you'd be better off playing a game that is made for that" usually rings hollow. What about you?
Super hollow, if only because I see it all the blasted time and it gets tiresome. A dedicated D&D player who wants to sample other genres and play "space-flavored D&D" or "pulp-flavored D&D" or "modern-flavored D&D" can already do that—provided one is willing to play either an OSR game or a 3e-based d20 System game. In fact, it's hard to think of genres that haven't been given the D&D treatment at some point within the last twenty years.
Of course the end result isn't going to be a different RPG. It's going to feel like D&D. And that's the whole point! If what you want is a heroic adventure game with experience levels and character classes, why would you look anywhere else? Which is why this attitude—
This. You can send D&D characters into space, but it remains D&D in space.
If I want to have a starship battle the D&D rules don't cover it, but other RPGs do.
—baffles me. As if "D&D in space" were somehow undesirable (or difficult to achieve given all the extant material out there—there are no fewer than four OD&D-compatible sets of starship rules and probably more for 3e sitting on my bookshelf right now). No, it won't be Star Trek or Star Wars, it'll just be star-flavored D&D, but why wouldn't a group of D&D players want that? Sometimes the goal isn't to feel like you're in a weekly Trek episode or a bad Star Wars sequel movie. Sometimes the goal is to earn XP and gain levels as a Jedi or a Science Officer or whatever.