Commercial kits are great - an easy solution for those who want a buy it and be done solution. But:

Building your own 72 hour pack has these advantages:
1. Almost always a better, bigger pack
2. You can personalize the gear for your situation
3. Most pre-stuffed packs have little room for anything else. Wanna bring a jacket and your laptop...then you're out of luck.
4. You can avoid toting cheap, heavy gear by not adding it.
5. It'll force you to think...