the reason why bay was liked more than mcab, at least by me since i can't speak for others, was due to:
1. bay had a horrible lineup around him last year as well, but produced. i had no idea what mcab would do with a horrible line up around him.
2. walking isn't just completely dictated by the pitcher. a hitter knows how to walk as well. bay knows. mcab, before this year, seemed like a freeswinger (which is usually a bad thing, unless you can sustain a .330 avg or something with .500+ slugging% like vlad, and admittedly mcab). so basically, since mcab is a freeswinger, if pitchers give him only bad pitches to hit, will he get frustrated/impatient and swing at them? that would lower his avg, increase his Ks, and result in overall worse #s than last year. however, he has adjusted, and is on pace to walk like 80 or 100 or something (too lazy to check), which would be more than he's ever walked before by almost x2. if all this walking is due to a conscious adjustment made by mcab, and not just a fluke, then i think mcab will be fine.
3. i thought the lineup around mcab would be much worse than this. hanley ramirez, and especially willingham, have surprised me this year. they don't look all that bad at all, certainly better than the pirates. but my point is that before the season started, i thought the florida lineup would be just as bad as the pirates', and thus had mcab ranked low.
as for your question of why are people saying bay is a buy low player, he's had .900+ ops EVERY SINGLE YEAR he's been in the majors. that's insane. he knows how to hit, walk, and mash. don't worry about his hitting. the only question is whether he'll steal bases or not. that's up to you to decide. personally, i think if the lineup around him doesn't produce, he'll start creating his own runs by stealing more. if the lineup around him produces, he'll be good for 100+ runs and 100+ rbis. and in your specific situation, since you're the bay owner and not trying to buy bay low, there's no reaosn for you to trade him now since you won't get full value. if someone's offering mcab or hafner, then sure go for it. but i'm guessing the kind of offers you'd be getting for bay now would be a hideki matsui or so.