GarageBand can't play external MIDI instruments. It can't record more than one track of MIDI (input from MIDI controller) at one time. It can't record more than eight tracks total at one time. Automation is limited. It cannot print scores/sheet music. I'm sure there are other things, but these are some of the major ones compared to Logic, etc.
If you are an advanced user, you many need some or all of these features. However, if you are just starting out, or need something fast and simple, the lack of these complicating features make GarageBand
better than the "more pricey music software." Here's a for instance. If you want to record MIDI, with GarageBand, you just plug in a MIDI device (probably thru USB) and it just works. You don't have to tell the MIDI device what channel to output on, OMNI mode or whatever, etc. and you don't have to tell GB what MIDI channel to look for data on and route that to this track.
When you record MIDI, that's called a Software Instrument in GB, and you simply select the built-in software instrument sound you want it to sound like. You are limited to what you have in GB (and I'm including 3rd party Audio Unit plug-ins you can buy), but these kinds of things sound incredible nowadays.
Choosing a built-in software instrument sound is WAY simpler than routing the recorded MIDI data back out to your hardware MIDI instrument with all the selecting channels and etc. And, you can even do all your work on your Mac with nearly nothing else. You can edit your mixes sitting in the park! (Yeah, you should check your mixes thru various speakers in different settings, but you get the idea.)
Bob LeVitus (Dr. Mac) started out working in recording studios, so he has experience with the real thing. He's said that he has tried every recording software for the Mac, and they were all too complicated and he never kept using them. It was just too much work to record and get an idea down. Now with GB, he can do just that. He started using GB when the first version came out, and still is using it today. I can attest to what he's saying. I've used several recording software programs, and they were a pain. GB makes it simple, and like someone said, it comes with your Mac or as part of iLife making it really inexpensive.
iMac G5 20", MacBook Mac OS X (10.4.7)