Well, each house has a quest or two related with it (have to do the jarl's quest in order to buy it), so buying them is optional but it helps "complete" the game, purchasing all the houses.
Just an FYI though, the blood and ice quest is horribly, horribly glitched in multiple ways (the quest in windhelm where you have to find the killer of the women), so make sure you save before doing that one so you can re-load, as it's known for glitching bad.
I also saved the main story for quite a while through my game. The main story is pretty good, though, in my opinion. Also, you can only get a fully powered FUS RO DAH (unrelenting force) shout through the first 2 quests or so from the Greybeards, which is the most useful/powerful shout in my experience.