If you don't need arrangement flexibility, MainStage can handle all your backing track, loop, and click needs with the Playback plugin!
Here are a few tips for using Playback:
1. MainStage handles .aif files better than any other file type for playback.
I've tried all different file types for tracks. MP3, WAV, etc and .aif files definitely put the least strain on my CPU. They also contain tempo information which allows you to adjust the tempo of your tracks as needed WITHIN MainStage. Garageband, Logic, and every other major DAW can export .aif files (in Garageband its the "uncompressed" option).
2. If you want to change patches within a song, put your Playback plugin for that song at the set level and drop your patches into that set. This gives you the ability to jump from sound to sound without interrupting your playback.
3. The "Klopfgeist" click in MainStage might take some tweaking to work well for your team.
At first our team had a really hard time discerning the tone of the stock metronome in MainStage. I ended up adjusting the note values for the accent and beats which worked for our team. You can also designate another plugin like "Ultrabeat" to provide your click sounds or you could use an instance of Playback to provide a prerecorded click. Or check out this tutorial to learn how to change the sound to anything you'd like:
4. MainStage can jump around in arrangements IN THEORY, but in practice it works most reliably when playing front to back.
There are some folks I've run into who successfully change track arrangements in MainStage on the fly by using "markers" within instances of Playback but I've never had enough luck with it to risk using for a live service. Your mileage may vary, but if you really NEED that flexility then Ableton Live might just be worth it for you :)
5. If you're running a single stereo track for playback with guide panned one way and tracks panned the other (which is what I almost always do) then one trick I've learned is to mix that guide track SUPER HOT when putting it together in your DAW, and then use the pan knob to dial it back or up to the volume I need. Then I don't have to run two instances of Playback, which reduces the stress on CPU!