Some times trash mobs can be a nuisance, but that is only the result of poor level design.
Trash mobs can add context and enrich the environment pro providing a nice excuse to explore some paths that may not be mandatory, but offer some nice rewards as vanity drops or pure lore, as for example a very short video.
So why the hell does Omega have zero and rabanastre has nearly that...? I enjoy properly used trash mobs and a good level design, but Omega is a bloody arena and rabanastre is a corridor. Kind of lame...