1 - They are overpriced badly programmed pieces of crap that bored middle aged housewives and teenages buy and fund making them lucrative and diverting money away from good projects.
2 - Who the hell wants Farmville and Runescape earning hundreds of millions a year, why can't GOOD developers who actually make something get this cash. The 'Casual' games don't push any envelopes or expand any technology. The Wii is a frigging step back in power, it has a frigging 725 Mhz SINGLE CORE CPU. Ever been pissed when a game doesn't support crossfire, 64 bit or 4 cores, consoles like this earning billions are the cause. Why even frigging bother making half-decent code when you can slap dreamcast level technology onto a console for a tuppence and make a killing
3 - Casual games are, obviously, casual. Sure the Wii is fun but I doubt many people play it 24 hours straight. The greatest modern games can keep you playing from 10 am to 10 pm the next day, unless you have some serious issues I doubt you could stay on Wii sports all night.
4 - In comparison to most titles they are incredibly dumb with hardly any variety in tactics or playstyles, it isn't 'true' gaming.
5 - Like it or not they hurt true gaming, no big companies are going to bother paying big bucks to develop high tech engines when friggin' peggle will make you near on $1 billion. This is just my opinion but a lot of the recent releases seem to be getting less and less challenging and more 'casual'. Modern Warfare 2 was filled with cheap, easy to use guns and gimmicky killstreaks with poor balance *cough* Model 1887's *cough*. The recent C&C games have been a doddle compared to old school Red Alert Counterstrike missions which were hard as nails, inevitably casual hurts original gaming and that's not a good thing. It's like sacrificing the Mona Lisa for a bleedin crayon painting.
As for Casual games improving the reputation of normal games I think that's not happening at all. People are surprised to hear that I play many PC games and call me a game addict etc but are totally happy with themselves playing just as much on Wii's and Casual games. People don't see 'Casual' games as games, they've just been integrated into some giant lattice of douchedom.
There's also the point that I've never seen someone go 'Holy crap' at a Casual game, they generally just have a glazed hypnotized expression and play it mindlessly. No casual game has the sheer wow factor of extremely High-tech modern games, people see you playing Arma II and Crysis on maxed out graphics and just gawp; I think people enjoy 'casual' games simply because they have been too scared to properly game and only just realize how good it is. There's still a lot of stigma and prejudice out there which makes women want to stay away from High-end PC's and games despite the fact that it's very easy to learn and sort out.