Let's say you opt or are chosen for area, you qualify, you validate.
12 months down the line, you realise area isn't what you thought it was, or perhaps area doesn't like you - unless there is a caveat that allows you to retrain on ADC (because you will have l;ost the ADC rating after 12 months if you don't validate it), and APR, you are well and truely in the brown stuff.
However, if you do the ADC and APR, but don't like working for NATS after the validation, you can walk straight into a job at a non NATS unit almost immediately.
The above scenario isn't by any means a common occurence, but it does happen, and should be considered carefully when or if you have a choice of which course to do.