Keep in mind that by earning higher levels in a particular program you also earn a higher bonus mileage rate. So, instead of 25k in UA + 25k in DL, you'd have 50k UA + 6250 UA for the 25% bonus in RDM.
As for
- Being able to book economy award travel last minute, make changes, and cancel/redeposit without huge fees.
Take a look here for UA and also take a look at whatever other ones you want to compare to:
http://www.united.com/web/en-US/cont...ticketing.aspx
For last minute awards, having miles in more programs will give more flexiblity. But only if you can accrue enough in each program to have the required amount for your intended trips. Having 10k in two programs isn't all that useful.