I've yet to see a convincing argument in favour of the Mac as a gaming platform; ... The Mac does not qualify even as a has-been gaming platform, it's a never-was. Wakeup and smell the coffee.
There are about a million arguments in favor of Mac as a gaming platform. The reason why it isn't is mostly because of history; Windows became the dominant gaming platform for home computers, Mac did not. There's a million reasons behind this, some technical, some business strategy, some mistakes by Apple. However, it is perfectly capable of being a gaming platform on a technological level. Let's not forget that before Halo Bungie was primarily a Mac developer. The first Marathon game, as far as I know, was never officially ported to Windows. Not only that but Blizzard still to this day supports Macs. They even updated games such as StarCraft which were originally written for OS9 to work natively with OSX and Intel macs. Another point to note is that the iPhone is gaining ground as a mobile gaming device. This may help game development on OSX by proxy, although personally I doubt it.
As for Tribes, it was never made for Mac, nor intended to be ported I believe. Now there's no reason to port an 11 year old game. Not that it couldn't be done since the Darkstar Engine did use some cross-platform APIs such as OpenGL, but it's a bit pointless more than a decade later. However, the descendants of the Darkstar Engine which GarageGames currently develops are cross-platform. The newest release, Torque 3D, works on both Windows and OSX. Legions, which was built on TGEA, is also cross-platform. So if you want an argument for Mac as a gaming platform, ask companies such as GG and Blizzard why they support it.