i would see it important to bounce down tracks (especially) if your the recording engineer and not mixing the songs.
when tracking the songs you create tones and sounds that you think need to be in the bands music and is best for the sound they want, and what you want.
this may be more applicable to indie or other types of music that are less cut and dry then metal, but the theory still applies.
say you are recording drums and you have this great room and throw up 3 room mics. or you have two different mics on your guitar cab.
in your session you may blend those sounds together to create a tone or ambient room tone that needs to be used in the mix.
rather then sending the mixing engineer a bunch of files that he may not blend the way you wanted or he may even scrap some of them and just use one, you can blend them together, bounce them down into a stereo or mono track, and send them out for mixing, in the way that you intend, that way you vision is brought across in the mix. (may be a bit redundant in my word skills, but thats my theory on it all) i also like to keep a very analog mindset while working in the digital realm, so that may be a part of it. so also i like to approach recording with a band thinking, they have this and this, and all that, i dont want to use more then lets say 32 tracks. so i may bounce a few things here and there to consolidate my number of tracks. i don't believe that just because you can have unlimited tracks that you should actually do it, or even have 50 guitar tracks. sounds like something my friends mom would do. (hoarder)