I don't recall I've ever said they were bad...
Just not as good as some others. With sample ROM as cheap as it is now, it's hard to find ANYTHING that sounds 'bad'!
Now, if Yamaha started putting the same sounds and kits from their DTX drum products into their arrangers (like Roland do with their very popular V-Drums line), I'm pretty sure you would be able to rave even MORE about how good the drums are!
Anything that passes muster with REAL drummers is usually the thing to shoot for, if you are after realistic drums, at least...
But reality isn't all it's cracked up to be...