I also don't expect you'll have to repair your armour half way through a dungeon all that often.
People spend millions of gil upgrading from a Hauby to a Hauby +1 in FFXI. Given that armor apparently degrades over time, keeping it in tip-top shape (if programmed will, I'm looking at you SE) would be worthy of bring a few master crafters along. I bring up the Hauby and Hauby +1 comparison because let us say that you are doing a party's worth of Guild Leves i.e. 2 x 15 some leves. Now let us assume (because unfortunately until the game comes out we have no way of knowing for sure what tweeks SE will make to the degradation rates) that your armor degrades from the effective stats of a Hauby +1 to a Hauby by the time you get to Leve 7 or 8. Then the gain from having a crafter in the party would be worth the perceived difference between a NQ and HQ Hauby. This is, of course, assuming that armor doesn't degrade faster than that, giving crafters even more potential benefit to being in the party.
As to your other point, I would think that repairing low level gear would be relatively easy, but as the gear gets better and better, the expertise required would increase proportionally.