the Wilhelm scream is maybe a little different than other overused sfx, in that sound editors (particularly our friends up at the ranch) are obviously using it for fun, as an "in joke", whereas that dolphin chirp gets used partly *because* it's so cliche ... it immediately communicates the idea of "dolphin" even when no marine mammal is present on screen ... which is why, when i wanted to make a "dolphin" ringtone, i used it too!
could i have used some other dolphin effect? maybe, but a) i personally don't have access to any dolphins to record, b) even if i did, getting them to make noise on cue in a quiet environment maybe ain't as easy as it sounds, c) source material for editing is what those sample libraries are for, BUT most importantly, d) other samples i tried just sounded like wierd chirping noise, unrecognizable as "dolphin" without visual reinforcement.
so maybe using a cliche sfx is not always a bad thing, and maybe it's only obsessive audio geeks who ever even notice it anyway. maybe it's the same kind of thing with synth patches and other musical sounds ... when the producer says "i want it to sound like the '80's", you know that means "gated reverb on the snare"; or "make it sound like ELP" = "portamento square wave with chorus". maybe _all_ new and interesting sounds evolve from innovative to stereotype to annoying to useful cliche ...
p.s. i recently found another instance of the Wilhelm scream i haven't seen documented anywhere else -- mixed in with the explosion after "say hello to my little friend!" from Scarface!