The most common complaint raised against college football recruiting services like Rivals, Scout, and so forth is that they're simply based on which schools are recruiting and offering which players, not on any actual talent evaluation. While there's an obvious hole in that argument as-is (why should a list of unproven prospects not factor the level of interest each prospect has raised?), it turns out football coaches themselves don't have quite the same objections.
Some examples? California Golden Bears coaches check Rivals to put together lists of players to scout, which they then narrow down in a more conventional manner. Larry Coker's Miami Hurricanes staff used recruiting services to a disastrous fault, a trap Mack Brown's Texas Longhorns staff seems to have fallen into.
The fact that two programs abused these services isn't an argument against their usefulness -- it's an argument against laziness.
While putting major weight in online rankings is clearly inferior to investing eyeballs in actual scouting, you'd be naive to think even the most successful coaches don't pay attention to these sites. Auburn's Gene Chizik has admitted coaches are very aware of how highly their classes are ranked.
The call to completely ignore all recruiting websites comes more from fanbases whose teams rarely see their classes rated all that highly. While you can't approach Rivals and Scout without a grain of salt, the data shows that this stuff matters.