I could easily build a bomber list that could win without Rhymer. Rebel players do it all the time. But why wouldn't you take Rhymer? He makes your list more effective and doesn't alter your plan of attack in any way. And as I mentioned, there are no Rhymer-specific counters. The only things you can do to stop Rhymer are the same things you do to stop regular bomber lists, except they're less effective against Rhymer because his strike range is so much longer.
But we don't really know how much of Rhymer success should be attributed to Rhymer himself and how much is the success of Imperial bomber lists in general. (I tend to feel that it's more of the latter).
As for the reasons to not bring Rhymer: to maximize AS output (bring two bombers instead) or to be able to divide your bomber force to attack two targets at once. Its not for every fleet, however I can see situations where this choice will be valid.
I guess I really only mean my statement in regards to Demo. I haven't felt much sting from Rhymer. As for Demo, I feel like I counter him pretty well already. I fought two Demo lists on Saturday in Marietta and lost to none of them. It's not that he's not good, it's just what pt106 said. If you know he's coming, there are things you can do to give you an edge.
People call him broken all the time, he really isn't. He is Good for sure but not the End all/ be all of Imperial squads