I think the fact of the matter is that people see '20 second respawn waves' and immediately think; "I have to wait 20 seconds to respawn??? OMG that svcks!!!!!" When in reality that's just not true, and having to wait longer than 10 seconds will be extremely rare.
I think you're right about the reason for peoples' reactions, but I wouldn't go so far as to say waiting longer than ten seconds will be
extremely rare. It won't even be rare, considering you have a 50% chance to have to wait more than ten seconds. Still, even that's not a big deal, and it's not like that will happen all the time. As others have said, waiting the full twenty seconds will only happen 5% of the time.