Protection against 51% attacks is a concern.
51% mechanism is what allows to challenge the status quo, thus enabling competition for power.
Just sitting on your money in PoW does't give you control over the system, just engaging in the competition for control doesn't bring you money as mining is normally a break-even game. Thus money and control become orthogonal, which results in a dynamic system without asymptotes. In PoS you get both in a nice single package, which saturates fairly quickly.
Hybrid systems are more tricky beasts, thus they need to be studied closely. I'm concerned that saturation in PoS aspect of the hybrid will at some point begin to negatively influence the competition on PoW side.
51% PoW mechanism is what allows malicious agents to challenge the legitimate miners, thus overpowering them via a hashpower coup d'etat.
Do you really think that the legitimate miners in Bitcoin can prevent a government from forcing
their rules on the Bitcoin network? Mind you, the NSA had a $10.7 Billion USD budget in 2013. The only way to prevent such actions from external actors is via PoS.
If you are interested in using PoW as a distribution mechanism and PoS as a security mechanism,
NXT's monetary system offers such a solution.