What's your experience on the endurance of MBS's on baitcasters?  Do the magnets tend to hold their power over time or do they weaken?

A permanent magnet can loose it's magnetism if subjected to a strong outside magnetic force, a strong electrical field, or, in some cases a very high temperature. A temperature well beyond what the rest of the reel can withstand.

 

A typical permanent magnet can loose about 1% over a ten year period.

 

I think you have little to worry about.

Agreed.

My first baitcaster was a daiwa 100h and it performs flawlessly like the day it I got it almost 10 years later. Its on my C-rig rod that I use everytime I go out, most of the day.

I purchased a Daiwa Zillion when it first came out.  That was about 7 years ago.  The breaks on it are sometimes questionable.  It has always worked best with heavier lures but it seems the older it gets the more I have to keep increasing the breaks in order to prevent backlashes.  When I first purchased it and was fishing a weightless senko, I was setting it to 3 or 4 but now it is more like 5 or 6.  Don't get me wrong, it is still a great reel but I do think the effectiveness of the breaks has weakened over time.