Martinez can't play 1b without his knee tendinitis flaring up. I can only imagine how much time he'd miss playing 3b regularly.
Luckily I haven't had significant knee problems myself, but I would think that running around in the OF would place more or as much strain on bad knees as the pivoting, etc. required in the infield, no? Wasn't that part of the reason Berkman moved to 1B after Bagwell retired?
Not that I am advocating moving Martinez to 3B, a position he has never played...