People like to claim that Mac users do not pirate as much proportionally as PC users. This may be true, but I have yet to see any evidence of this. I think it's wishiful thinking. I would love to see any actual report, chart, anything...
I can assure you that there are gobs of Mac pirates out there. I used to be one of them. The community is alive and well (and growing rapidly). There are PLENTY of tools for Mac users to pirate. Not that I'm endorsing it.
I know dozens of people with nothing but pirated Mac games. Yes, it's shameful. I could name names, but I'm not going to. Mainly because there is no point given that nobody would know who I'm talking about. I can name myself for one (formerly). I am now all legit. Of my peers, I am in the minority.
I think the developers (porters) should absolutely be concerned. The Mac game market is so pathetic to begin with. Any additional hit from piracy will be devestating. And yes, it is a legitimate argument IMO. And when games go away from Macs forever (the big games that is), I will laugh in the face of all the pirates who made it happen. If you don't believe me, go to versiontrakcer.com and read the reviews of Halo for Mac. There are several idiots who say right there that they will heretofore pirate all Macsoft games simply because MacSoft provided an update that requires the CD in order to play. Yeah, it's annoying, but does that somehow justify rampant piracy? These people deserve what they get.
The PC market is a copletely different story due to sheer volume. You can still make money even if half your audience pirates your game.