I had a standard Craftsman riding mower (46" cut I think) for maybe 7 years or so and it looked/ran great when I sold it. I didn't have any real problems with it. I mowed my hilly 2 acres or so at the time with no trouble. I sold it because I wanted something new.
Moral of my particular store is, it doesn't HAVE to be commercial grade to last under reasonable condition and good maintenance. Of course, you will notice a difference in cut quality and speed when you get a commercial mower. Is it completely necessary to have commercial for normal home use? No. Is it still better and more fun to have a commercial mower? Yes. :-) Naturally, price is a big factor. My eXmark cost about $5k more than a regular riding mower I could have bought. I will also add though, I think my eXmark will last an easy 20 years if I decided to keep it that long. I can't see myself keeping it for more than 10 though because I'll just be itching for something newer at that point.
Gas usage...I think the eXmark I have now uses maybe 15% or so more gas (guessing)...but I also get done MUCH quicker. It doesn't slow down going up hills like the Craftsman and Cub Cadet I had did. It'll fly up hills.
Photo Gallery of the 'Guy Stuff' (eXmark, Stihl, etc): GALLERY LINK