How do you know Ortiz will "suck out loud" playing defensive 1B? They've always used him at DH, but when I have seen him at 1B, he hasn't been half bad. He's no Derrek Lee and likely not as good as Albert Pujols at 1B, but who knows? Ortiz has easily taken over 1B in interleague play and, those two games of the WS.
They are DH's not because they can't field, but because the team has better fielders that hit well enough to justify a lineup placement. If the Sox make Kevin Youkilis or Manny Ramirez a DH, it doesn't mean he can't play his position (in his case, definitely below average), but rather because they decided to use a better defensive player that day (like the now injured Kapler). Just because the Sox don't want to hurt themselves by playing a defensively capable yet inferior 1B like Ortiz over Olerud at the position when they can place Ortiz at DH and Olerud at 1B doesn't mean Ortiz can't play 1B.