I tried requests on some trips, and every single time they were ignored. And these were very simple requests, too! And yep, on some of those trips I wound up with rooms that were in pretty bad locations. I find that begging at check-in can help get a better room, though!
Rattling some of my failures off my head:
3 weeks ago at WL: I requested a balcony for privacy and security reasons; received ground floor directly next to an entrance/exit door. Threw hissy-fit at check-in and got very nice 3rd floor balcony room.
POR - twice requested AB; instead both times got the farthest room from everything in the back corner of mansion Parterre Place.
CBR - at check-in, I asked for anything but Trinidad South. Got Trinidad South.
CSR - asked for Ranchos; got Casitas. Beg and begged, and got Ranchos.
I find requests are just wasted bytes in the computer - they are meaningless. No one reads them.

I am officially done ever bothering again with requests! I will probably always be booking a better view/category for now on (no more standard views), just so I can get better located rooms.