Need? Never. I need water, food, oxygen and shelter. But want? I think some of us want the crunchy bits to better reflect our own style of campaigns and play. My house rules (not including races/prestige classes/etc.) is only a couple of pages long and reflects changes I felt added a unique flavor to our campaign world and fit our style of play more. I've seen it taken too far by some groups (at least I felt that way) and in other groups I could see how a simple change would have made them all so much happier. So, yes, I think they do some good. The trick as I see it is this, don't change things just for the sake of changing them, but if there is a REAL reason, beyond "this makes more sense to me", then approach it cautiously.
My argument against "this makes more sense logic": I hate reading, "well I'm a martial artist so I know how fights really are", or some other such nonsense. Martial Arts have rules and even the least restrictive among them involves very little attempting to kill a 9,000 pound fanged beast with a large pointy metal object and a fireball. I've taken several years of martial arts and can honestly say they really teach very little of actual fighting. Now I get in fights alot at work and well I can say this most end up in massive grapple on the floor and none are the same. This is my example of the fact that everybodies experience differs greatly and so what makes sense to you rarely reflects the actual truth.
I prefer to only house rule for flavor. Sometimes I house rule for simplicity or to "correct" a balance issue my group and I have noticed.
Drexes