Its pretty simple really, a big part of PvP is all about picking the right target and in the case of multiple similar targets such as say 2 white mages for example, you might want to take out the weakest/strongest healer first. For example, being able to view their gear is the easiest way to identify the targets potential strength such as the stronger healer wearing lvl 100 gear versus the other wearing level 70. With glamours, it would not be possible to identify factors such as this.

As stated above, this is common in just about every MMO with competitive PvP.