Real quick - could you explain a midi band vs. the more obvious live band?
I'll field this one! Real quick: MIDI composing is a way to use computers, samplers and synthesizers to play back the music you create. Handy if you don't have an orchestra on hand! Often composers working on their own will use MIDI sequencing to do most of the music. Layering one or two live instruments on top of it can add a more "human" element to it which IS noticeable (even to those who can't tell you what they're noticing). Which is not to say that MIDI recordings can't be expressive and realistic, but that depends on the sample libraries the composer has.
Less quick: MIDI is data. It can be created by playing an instrument (keyboard or other MIDI controller), sequenced by hand or a combination of the two. In addition to basic data (when the note starts, what note it is, and how long it lasts), you can record any number of "control changes", which can modify the sound in an expressive manner. They don't need to be recorded at once. For example, I'll play a passage, recording it. Then I'll play it back, recording knob motions to add expression. I'll do several passes on each part; depending on the complexity of the piece, I usually have over 30 tracks of MIDI at once.
None of this data is sound. The next part is to assign the data to an instrument. For electronic textures, a synthesizer is usually employed, which creates sound from basic electronic components (I could write a book here, but the important thing is that sounds are constructed, rather than pre-recorded). For organic textures, which is to say instruments that sound like real life instruments, the composer would use a sampler. A sampler takes a tiny recording of an instrument, and essentially loops it for playback for as long as you hold the note. You can raise the pitch, which essentially plays the sample faster, so that it sounds like the instrument is playing the note that you want. It gets more complicated from here, but that's the basic gist.
The advantage to doing this is you can say "what if that piano part was played by an oboe" and easily switch the instrument that the MIDI data is playing back. It's also cheaper because you don't need to hire an oboe player to come to your studio if you don't happen to play oboe. However, it's good to settle on the arrangement before you add expression; every instrument is played differently, and if you want it to sound like said instrument, you need to work the MIDI to recreate the sort of expression that instrument would have.
I said cheaper, but definitely not cheap. Good sample libraries can cost hundreds, even thousands of dollars. Synths aren't cheap, hardware-wise, though there are some interesting free plug-in synths available on the internet. Let alone the time and experience that the composer needs to put in to end up with decent results.
Anyway, that's the basic concept. Composer and MIDI for orchestral score work, find a band for incidental music (or end credits). Not hard and fast rules (one of my favorite scores is Neil Young's score to Dead Man...almost entirely solo electric guitar), but guidelines to get you thinking along the right lines!