For me, spin-offs can make me view the main games in a new light.. like for a pretty generic example, the Mystery Dungeon or Ranger series, or even Colosseum and XD are completely different concepts but give me a new outlook on the whole Pokémon world. Spin-offs are pretty important into providing players with new canon and expanding on their creation.
As for what I prefer... I would say I prefer the main games because they give us players most of the main content for what the spin-offs are based on. The main games are the main dish and the spin-offs are the dessert (lol)
But if I had to choose between getting games in Pokémon's main line of games and their Colosseum and XD line of games then I'm all for the latter!