The first is that it can cause scaling issues by snap-shotting procs. As was mentioned earlier in this thread, it does put direct-damage at a huge disadvantage compared to those DoT people. Add on top that it can cause some crazy results and completely skew damage spreads, it's just asking for trouble when it comes to balancing.
The second reason, and probably the bigger one as mmokri is implying, you need an add-on to play effectively. With snap-shotting in place, you would need to know the exact value of bleeds depending upon multiple combinations of buffs (such as RoR, Dancing Steel, another trinket proc, and any combination of the above... toss on DoC for kicks, too!)... there's a reason we use an add-on or WA to track the strength of bleeds, because it really is complicated. Even with the added help, how many people know at which threshold of bleed values you would overwrite Rip/Rake compared to the new strength of the bleed you could potentially put it? While I think some of us could handle juggling these variables even w/o the use of something to track bleed strength, it's insane to think the majority of the WoW population could.
All that being said, I don't think the complexity of the Feral rotation should be how fast someone can do math on the fly... and this is coming from someone that loves doing math. Imagine people that hate doing math... I've been hit by flying Calc books in the name of my spouse hating math.