I've never been a fan of relying on a proc for anything. Even maining WHM from 2.2-4.0, if I had to rely on a crit to save someone I'm healing, several things have gone wrong already. Hopping for DH to proc can have great and also terrible consequences. Sure, you can get it to 13.3% without the penta melds, but that's still not great. If you get the average and 13 proc's out of 100 swings, that's just not a ton of extra damage, since tanks skill combos aren't major damage anyway. Sometimes you'll get a lot more procs, sometimes you'll get even less. I'm a fan of the certain damage increase for every swing with Ten, and get the passive benefits of the mitigation and extra heals.

Knowing I'm getting the benefit per swing (with Tenacity) is better to me than hoping my DH procs during a DPS check.

I also had another question in the OP. If healing is benefited by Ten, does that mean that Adlo's would get the benefit *2?