I use them to top off high level pets for EXP quite a bit (depends on the tamers that week). They provide less experience than Northrend/Cataclysm tamers (and way less than Pandaria tamers), but more than wild:
I haven't fully vetted out all of those values like I have for Pandaria/Cata/Northrend, so they might be off by a point in a few places, but those are the equation result values for the Celestial Tournament. This does not apply to the Celestials themselves, just the undercard tamers.
More info at viewtopic.php?f=10&t=8829
EDIT: In terms of teams I do a carry pet on:
Taran Zhu: Very RNG dependent, but usually pretty reliable. I use an [S/S] Death Adder Hatchling and Stitched Pup
Chen Stormstout: Very easy, but slow. I use a rabbit for Tonsa (you can replace this with any pet you use for Tonsa) and Chi-Chi for the cricket and elemental.
Wrathion: Kinda slow and a little RNG. I use either a Snail or [P/P] Emporer Crab for the undead, [S/S] Flayer for the two dragonkin. Crab seems to work better
Lorewalker Cho: Somewhat RNG. Feline Familiar + [H/H]Scourged Whelpling.
None of those teams are 100% the best. I've only experimented to get some data points for that table. I will say while the EXP is kinda meh, the ability to go in, due 2-3 carry pet battles, leave, heal and repeat is kinda nice. I'm not sure if it is faster or slower than doing wild pet battles, but I am leaning towards being faster.
I know I didn't post full strats. I don't really have the time and I mostly have done it by feel, so it would be off the top of my head anyways.
EDIT2: For reference the relative experience multipliers for tamers: