As already mentioned, Shimano's newer, higher end RD's don't have barrel adjusters. The body of the derailleur isn't made to accept one, so for those derailleurs, you'll have to do without. I think the weight savings and the fact that the shifters from the same group(s) do have barrel adjusters is the reason Shimano left them off. They were doing away with the redundancy of two barrel adjusters along the same cable.
Personally, I like a barrel adjuster on the rear derailleur. Heck, I wish there were barrel adjusters on front derailleurs. But as already mentioned, you should be able to manage with your barrel adjuster on the downtube cable stop.
I've got a Surly LHT with that setup (XT RD w/ no barrel adjuster on the derailleur, barrel adjuster at the downtube cable stop), I can fine tune the cable tension just fine if necessary.