iLife philosophy says no MIDI out
I think the reason there is no MIDI out is because the iLife strategy, is to manage your digital lifestyle on your mac and integrate all iLife apps so that what each one does, is accessible to the other. Not that you can dump iDVD to iPhoto but...heh...media conflict there.
This whole strategy lets you bring your life TO the Mac. Apple wants you to have your Mac as the destination with the ability to archive to cd, dvd, paper etc. MIDI IN does this; plugging in a guitar does this and so on. The mix-down function of GB satisfies the second requirement. It makes the music accessible to other iLife apps, since it dumps to AIFF.
By having a MIDI OUT, you are then taking people AWAY from their Macs to other MIDI devices, for which Apple cannot control the user experience, which iLife is founded on as a core philosophy. Apple doesn't want to have to ever issue a 'GB won't play out of such an such a keyboard because it doesn't support GMS, has different patch sections which controller ## must trigger to do a bank cfhange blah blah blah' knowledge base article I'm guessing.
It may be because I struggle grasping MIDI and patch selection on my Roland XP-60, but I despise being invited to send some controller number and data number to get to patch 43 in section C from a MIDI app that doesn't have a sound list in plain english. I'm trying to get away from all that stuff. GB will help me delve into the world of softsynths. I don't see any easy way for GB to let users select a tone from a bank or keyboard expansion bay bank or GM set or what have you. Apple would have to constantly provide transparent patch selection tools and be issuing software patch databases all day long for every new sound module (with keyboard or not as a synth) out there. BLECH!
Also, with Apple's strategy to have the Mac as the center of our digital lifestyle, how practical would it be to have to change it to, "you can play your songs on your digital lifestyle Mac, only if you bring along that MIDI device there, that one there on and that big-ass digital piano there that you recorded those 3 filler notes that only sound good on that MIDI enable digital piano". I'm guessing Apple wants to keep your portable, for the "year of the laptop" type things too. A softsynth collection without any need to ever send MIDI messages outside of its tone generator to a U-haul of MIDI gear, is a sweet portable deal. Keeps your musical creative life on your Mac without strings attached (litterally).
It also allows this sceniario, "you're up till 3 am and you get this killer brainstorm. You jump in GB, toss the idea together really quick and click on 'save'. You then sleep 2 hours, get back up, yank your other band mates out of their sleep and you know they're too exhausted to run to the full MIDI setup you have a 30 min drive away from the hotel, so you just flip open the laptop and press play. They don't have to be truly conscious nor get dressed to hear your brilliance." By inviting a GB user to even remotely consider sending stuff out to other MIDI devices, kills spontaneous creativity and portable presentation unless you never leave the confines of your MIDI network. It's no longer your life, it becomes work. It's calleld iLife and not iWork heh.
GB is not a sequencer, it's an enabler. It's not a MIDI waypoint to push data along past a tone generator. It's a destination where Apple controls the plushness of your stay. And it does this with great quality, some of it professional quality.
I hope this makes sense and that I came off as having some kind of clue.