Proc vs. constant damage. 10% at that. Rend is 60 per tick for me, 5 ticks, 300 damage. A decent heroic strike is 300 damage. Hoping for a 10% chance of immobilisation is crap, most fights aren't lasting more than a minute and thats 6-7hamstrings used during that fight. You still aren't going to get to that 1/10 chance. Rend is cheap rage, cheap damage. Over that minute you're looking at 1200 damage from rend. At most I am seeing twinks with 2.2k (excluding feral druids which I have seen up to 3.1, exploit enchants) and that leaves only 1000 damage to do with white hits, heroic strikes, thunder clap, etc.
Nonetheless, my entire arguement was based on talenting Improved Rend (3/3) in the first place. If I hadn't talented that, I wouldn't be using Glyph of Rending.