Comments and questions...

The comments about select dining vs traditional dining pretty much sums it up. My spin/description is that traditional 6pm / 8:30pm dining is about simple consistency. Same time, same table, same wait staff, etc.
Select dining affords you the flexibility (time, tablesize, etc) but at the potential sacrifice of the positives of that consistency (ie. Wait team familiarity, potential waits for a table, etc). There are ways to get the best of both worlds in Select dining, namely, asking for the same wait team (potential wait for them though but quite doable if you work with the hostess), making reservations to limit your wait, avoiding peak periods, etc.

The purpose of the pre-paid gratuity for Select dining is that because you will be served by different wait teams, it's an easy way to pool and distribute the gratuities equally among the wait staff.
What's the concern about pre-paying the gratuity? If it's about potential lack of service, I doubt that would be an issue because it's a perk for wait staff to work in Select dining.

If you are in Aqua class, you have the option of dining in Blu (I think there's a Blu on Infinity). I don't think Blu has Select dining tho; only the MDR. So, I'm guessing you'll be forgoing eating in Blu to eat together in the MDR?