My Gmail account holds 64,800 messages. Of those perhaps a few hundred are of lasting interest, but by the time AIs sort out what they are we'll be long obsolete.
So, perhaps illogically, I like to hold onto the archive. That means a local copy.
I recently had an opportunity to test how well an IMAP archive approach works . I upgraded a machine I'd not used for a while and had to catch up on a year's worth of email. It took a few separate synchronization sessions to catch up, I think I ran into Google's bandwidth limits/transaction throttling behavior.
I ended up with 64,440 in my local "All Mail" synchronized folder, so about 360 have been lost in the bowels of IMAP. Perhaps they'll trickle in with future synchronizations.
The oldest message in Gmail online is dated 2/29/2004 , in Mail.app (Lion) it's 2/2/2004. Yes, that's weird.
To answer the post question then, I'd say Gmail archiving via IMAP is imperfect but not pointless. A C grade.
 It's not obvious how you can see this. If you mouse over the message count there's a hidden option to swap newest and oldest. It's not easy to get to show; navigating large email collections in Gmail is almost impossible with the "new" UI.
 IMAP archive is what Google's all-but-defunct Data Liberation Front recommends. I miss that gang, but there were of the Google- era. In the Google 2012 they're an anachronism.