This file could be stored on a server or in a user's Dropbox folder. It would allow users to pick an RSS reader freely (per platform), without losing and rebuilding all their data. It also keeps all this metadata in sync, one less worry for the developers.
Don't get me wrong. I totally agree w/ you and wish that switching is easier. That "cost of switching", though, is the primary reason I'm still using the reader that I am (and have been for years).
A rising tide lifts all boats. If something is developed that makes the entire ecosystem more attractive, then everybody benefits.
I'm guessing that most of the developers want you to use their reader and I'm not sure there's an advantage (for them) in adding code that makes it easier for you to switch to another product.
Sure there is... because I'm more likely to try your product in the first place if I know I'm not locked into it. "The best way to capture users is to make it easy for them to leave" and all that.