It's not like they plan for bugs to happen. The sad fact is there is no such thing as a 100% bug free MMO. Unless it's shut down then it's no longer buggy.
Bugs come and go, it's a fact of MMO's. One change could ripple and break other things that were never intended and some bugs can only be seen with thousands of players testing.
Anyone knows that, but the fact does remain that they are piling up and while they are adding content which is fine, old bugs that were at launch are popping up again. Also, your statement is rather unnecessary as anyone that plays a game realizes that.