PRL really has noting to do with Sprint towers. PRL stands for "Preferred Roaming List"...it simply updates your phone with any new roaming agreements Sprint has made so your phone knows which non-Sprint towers it can use.
There is also priority assigned, so if there isn't Sprint service available, but there are multiple non-Sprint towers available, it will use the one that Sprint has the cheapest rate agreemenet with.
I used to work at Sprint and 99.9 percent of the time, the PRL update identified new roaming agreements in other countries, or out in the middle of Montana somewhere. So realistically, it very rarely affects us normal users. So one single roaming agreement in the Bahamas can get pushed out in its own PRL update to all of us. Wohoo!
I have heard that in some RARE cases it can identify new Sprint coverage in certain areas.