There's no such thing as bad polka.
DMan's being annoyingly reasonsable and thoughtful, but I'd like to observe that non-religious lyrics in music were happening all over the 19th century: Lieder, opera, operetta, etc., not to mention Stephen Foster and ilk. And then there's "folk" music: There's not a battle in the history of Scotland but what there's a ballad explaining in endless detail who tolchocked whom and in what order, and then who ran away first. And we only seem to be talking about Europe and the US; I've no idea what goes on in the rest of the world. Finally, it seems a wee bit reductive to me to be focussing on ska punk and rap to the exclusion of everything else that's happening nowadays in popular music. Yes, this stuff wasn't happening at all thirty years ago, and that probably does mean something, but it's only one thing out of many that's happening now.
If you lose the "lyrics vs. melody" thing and replace it with "rhythm vs. melody", or better yet "groove vs. melody", you can throw techno in the box with rap etc. and then we may have something more interesting. But I'd still hesitate to make too much of it.
I wish hackthemainframe would share with us the definition of "melody" which excludes sequences of pitches that he doesn't enjoy while including the ones he likes. I'm not following the part about rhythm, either. Again, there may be a problem with definitions: Assuming that Pearl Jam qualifies as "grunge", the rhythm is usually so dismally simple-minded that you'd have to be stone deaf to miss it (and brain damaged to get off on playing it, IMHO). If you don't like it, you know, it's perfectly okay to say "this shit sucks". You don't need a musicological justification.
People have been dancing as long as there's been music, by the way. Not me, for God's sake, but some people. Don't ask me why.
The whole discussion almost seems slightly silly anyway. Y'all're talking about things happening in the last ten years, or twenty at most in the case of rap. That's a drop in the bucket, really, compared to, say, the so-called "romantic period" in the 19th century.