i typically give longwinded responses seething with detail, but i've only got two words for you here:
do it man... actually i just made this move in a league i've been up ~20+ points in for the last couple'a months, tho i'm down to 19 points now... i made a mistake in that i had buster posey and during his awful slump i cut him cuz miggy montero came off of the DL like gangbusters (hell, i didnt even take carlos santana cuz i had posey) and i've missed out on .432/7/24 or whatever, but i mean, right now montero had one solid ~.280/16/54 year last year and he was hitting early but he's a dead-ass hitter right now, like the whole diamondbacks is dead-ass team save upton who has had one of the most quiet ranked ~#60 overall seasons in fantasy history and has gotten that average up to .280 i think, shades of last year but not quite last year...
but yeah. i'd take a flier on wieters... what do you have to lose? a catcher hitting .100 over a solid stretch here? it's not like montero is some legendary masher who could burn you hardcore... he's proven he can get hot, but he's also proven he can get so cold that you gotta make a move... and wieters has the pedigree, lord knows, and he'll play cuz what else do they also dead-ass orioles have to lose? with b-rob back maybe the lineup gets a jolt and i reckon luke scott is tradebait so if he goes you figure that's even better hitting position for wieters, so midaswell give it a go. he's been solid since he came back from injury, with a couple'a HRs and ~9 RBI over ~40 at bats i wanna say? go for it.
I had the same decision to make in my money league that has small rosters, and I went through with it and dropped montero and picked up wieters. I think wieters has the higher ceiling and if Arizona is unable to find a trade partner for Chris Snyder, Montero could continue to see a big split in time if he doesn't find a hot streak. Do it.