I think it's funny that people worry about the weight of a guitar. No for tone reasons so much, but because they get tired. How fucking frail are you that having 7-10 pounds strapped on you for a few hours at most is a huge burden? It's not like you're actually carrying it. It's just hanging there. If that's a problem, you need to make some lifestyle changes. Eat better, get in shape, hit the fucking gym or something.
Yeah. I think some care about it for tone, too. The idea that lighter is more resonant...I guess there's some physics to it, but personally I never noticed that. I also don't notice a big difference between finish (and that's another reason for cost...many times US have nitro finish and MIM thicker poly finish). I actually prefer poly in a way because the guitar doesn't chip as easily.
A light guitar does feel nice, but whether it's worth all that extra money is up for debate, preference, and wallets.
I made an edit to my last comment saying that buying a used US guitar is a good compromise b/c you get it half the cost of new but also get the light weight, good components, etc. In the long run you might wind up spending less in repairs/upgrades if you buy a used US guitar vs a MIM.
Differences I can think of:
Better bridges (at least the P-basses)
Pots/soldering less shoddy
Lighter wood
Better pickups (i.e. usually, as Greg noted US Strats usually have thin/ice pick frequency laden pickups, and the MIM can actually sound better sometimes).
Paint (US fenders are limited to a few colors due to regulations, MIM get more colors but usually thicker finishes).
Usually US fenders have level frets from the factory, MIM many times need leveling.
"Feel", generic term, but a US guitar does feel different and sturdier in the hand.
US guitars hold their resale value.
Those are the main differences I've noted. I've played fenders my whole life and had them from Mexico, Japan, and US.
My suggestion is if going MIM learn how to read a neck well and look for uneven frets, etc, else you might get hit with luthier bills down the road where you're paying as much as a US guitar.