You do run into sharp diminishing returns. I'm not sure how exactly it's curved, but if you have, say, 200 strength, you might need to get to 250 to gain additional benefit. This is just a handwaving example, I don't have the actual numbers.
Note that in most of the MM games, there are non-combat benefits to having very high stats; for example, you might need a very strong character to pry open a sarcophagus.

If (and let me stress IF!!!) the stat returns are tiered like in MM3/4/5 then you start getting lower returns after 50 and start getting seriously less returns around 75-100. But I don't remember if 6 uses the same tiers so take this with a grain of salt...

Didn't notice that you specifically noted MM6. I don't think there are any non-combat uses for stats in MM6 other than that a few skill trainers have certain requirements, but I might be wrong about that (MM7 had the contests, at least, but Day of the Gods overpowers almost all of them).

MM6 has several stones with swords in them that require strength to remove.

Ah, that's right. Are any of those any good? I can only remember the one outside of New Sorpigal, which gave me a non-magical greatsword :p (I assume it's randomized, like any other chest).

Hello, this is exactly what you are looking for:
[url=]http://sites.google.com/site/sergroj/mm/mechanics[/url]

I don't think any were better than what I was using at the time.
I'm pretty sure they're not randomized, though.

I got varying results when I played but at least one was quite good. I think it was actually a unique artifact sword, which I used for a good while. Also, I'm not sure, but I think that some of the swords in stones require higher might to pull out, and that those are generally of higher quality.

http://www.angelfire.com/nt/bones/ has information in regards to the breakpoints as well.