Yes, I should clarify that I think it's great that IMAP support exists, that you can certainly backup your email relatively easily today, and that the Data Liberation/Takeout thing is great in general. I'm using email sync as the example here, but the benefits would really come from a universal (or more universal than today) sync, not just an easy email sync.
I just think Drive would be a great way to unify backup methods across the Data Liberation options. Gmail backup is already relatively easy, but some of the others are less so, and being able to turn them all on with a few clicks (rather than remembering to visit the web interface and exporting every few weeks) would seem to be a better solution. Ideally IMAP support would continue, and, even more ideally, the hypothetical Drive backup format for a gmail account would be some typical email client save format for IMAP accounts so you could (at least somewhat) transparently open the backup in a major email client.
All of this is certainly not trivial, and less so now that I thought through the above. If you treat the local "backup" as a first-class version of your mail by actually opening it in a client and then altering emails in there, drive could then in theory sync it back with the gmail account just with IMAP.
Some account data you'd probably have to specify is a read-only backup and not as read/write. For instance, exporting your Reader data gives you a whole json format export, as does exporting your g+ data, and I'm sure other products as well. I'd imagine it wouldn't be worth the extra engineering effort (and possibly a recipe for disaster) to support random changes to that json being synced back up to the cloud (if it was even data you had permissions to change in the first place...for instance, if you edited the text of a post you commented on, but it wasn't your post).
You could probably get around those by just explicitly labeling the subfolders as "Google+ backup" or something, so there's no expectation of changes there being synced back to your account. Should it overwrite any changes you made locally next time it syncs, though?
Luckily I don't have to figure those things out. Still a good idea though :)