Like others are saying.. It REALLY is a personal preference thing..
And personally I love carts.. T Techs don't run out more expensive, esp if you're all disposable.
Ok why do I use them?
1. Consistency (always the exact same set up every day, no variation of bending bars, rubber bands, etc.. always good ink flow etc)
2. Convenience (quick changes between needle grouping, less machine set ups etc.. more space on work station)
3. Quality (almost never find a duff one)
4. Less waste (not only in the needle/cart realm, but also in not using multiple machine bags, cord covers, packaging etc)
5. Time saving (setting up, changing over, cleaning up,)
6. Coz they're cool as fuck!
My 2 cents