It all depends. If something feels like an obvious cash grab, fans won't like it. If you get a spinoff when there hasn't been a new mainline game for the franchise in a while, fans won't like it - eg Metroid Prime Federation Force. I can't speak for Front Mission, but with Fallout 3 that wasn't even a spinoff. The issue some fans had was changing the main series from one gameplay style to another, which is a whole other can of worms.
Gamers like spin offs when they're well made games in their own right, don't detract from the main series, and are a genre that fits the franchise setting. Like I mentioned, Halo Wars seemed like a successful spin off, and more recently Gears Tactics was fairly well received.