Personally, I don't see this as being too tough of a call.
For one thing (and the main reason), I think the odds are that, if you offer Beltre arbitration, he's going to decline it. He knows there's a market for him out there, a market that includes Boston and Philadelphia, two big-budget teams with both a need and an understanding of how good he is. With both demand and competition for his services, Beltre's almost certainly going to get a few multi-year offers at a good salary, and some are going to come from better teams in better parks. It wouldn't make sense for him to take a one-year arby deal. It'd be one thing if we were, I dunno, Texas. But Beltre hit .254/.307/.411 in Safeco and .277/.326/.472 on the road during his time as a Mariner. The only reason someone in his situation would accept arby is if he thought he could re-establish some value, and Seattle's just about the worst possible place for Beltre to do that.
The other possibility is that Beltre accepts arbitration. The probability is low, but it's not impossible, so it has to be considered. If Beltre accepts, the team has two choices:
(1) Trade him
(2) Keep him
If Beltre accepts, then he's probably looking at a 2010 salary somewhere around $10-12m. Thecould then trade him to someone looking for a 3B, maybe with a little cash, in exchange for some sort of non-elite prospect, negating the whole thing. They don't come out with a draft pick, and they might have to spend a bit, but they do get something back. The only real trouble here is that the M's would have reduced flexibility until Beltre is out the door.
The Mariners could also elect to keep him at that $10-12m and fill a known roster hole. This is the worst possible outcome of the whole thing, but in this case the worst possible outcome nets us a good player at a reasonable cost with no long-term commitment. Beltre projects as at least a 3-win player, and that's a 3-win player with upside, as he's been a 4+ win player four times in eight years since 2002. The problem here is that his $10-12m salary would cut a huge chunk out of our offseason budget, but it would by no means leave us in an unmanageable situation. The FO would still have room to work, and they could always clear a couple million by moving Jose Lopez. So while bringing back Beltre would make things a little more difficult, it would answer a burning question, and it would still leave Zduriencik enough room to upgrade elsewhere.
Offering arbitration to Adrian Beltre has a downside. That's why there's a conversation. It isn't as obvious as offering arbitration to, say, Mike Cameron in 2003. But from where I sit, the downside is neither particularly detrimental nor particularly probable. Erik Bedard? Bad idea. Adrian Beltre? Go for it. He'll probably decline, and even if he doesn't, the consequence just isn't that bad.