Before they catered to casuals, the game had higher subs by far; numbers don't lie.
So to answer your question IMHO, yes it's somewhat ruining WoW because most of them want the content too easy and they have no room for improvement because they want the content scaled down to their skill level instead of trying to rise for the challenge. They put almost no effort into their character with keeping up with the most simple of things such a stat prio, and some casuals I've seen have mixed gear like a resto druid with half cloth/leather gear. I've set down and tried to help and improve many players, linked them to websites, addons, etc and they still don't care and act like they don't have time to help themselves. I raid in a 13/13 heroic guild and STILL hold a full time job fine, wife and my first kid soon and CASUALS are a disgrace when they act like they can't do crap because they are casual.