I have very little personal experience of working trials, and so I dont have much to base my answer on, sorry. My cocker has only done a small amount of working trials training, and thoroughly enjoyed it.
Because of their size a cocker will never be able to progress past UD, as after UD there is no reduction in the size of the jumps and scale for smaller dogs, so they will never become a champion and if you have aspirations to work the higher stakes other breeds would be more suitable, e.g. border collie. However, in my opinion there is no reason why cockers can not do well in the two lower stakes. I have heard that there are a few cockers in working trials, but I dont have their details to hand, sorry. I also know of some staffies and a jack russel that have qualified CDX.
The only trainer I know of round where I live (and he has a wealth of experience in the sport) said that he wouldnt bother with a cocker. He said they just get a scent/put a bird up and that's it, they are gone. He also mentioned their short attention span works against them, and so in his opinion people are wasting their time trying to train a cocker for working trials.
I think it depends a little on who you speak to whether they believe a cocker is a suitable breed or not. In my opinion it is much more about the individual characteristics of each dog, and the abilities of the trainer/handler. I believe my cocker has more talent for working trials than my collie, which goes to show that there are always exceptions