Feature - if the GM takes measures to limit the 15 MAD
Put in a schedule, and if the party waits eight hours while the wizard rests and prepares his new spells hey may be wondering what happened to that village they were trying to protect, and why are there columns of smoke rising from where it's supposed to be?
The Auld Grump, I have never had a big problem with it, and the one time they party assumed that they could just rest as they wanted... well, they learned otherwise.
For 3e at least, this completely ignores the easy availability of Scrolls, wands etc. (Scribe scroll is free to any wizard and low level utility scrolls, the worst offenders, are cheap and relatively quick to make).
Some specifics:
i]knock[/i] lets you pick a lock with 100% success and do it as quickly as a rogue can! And went through 2 locks per spell to boot. The wizard needs only a few scrolls (cheap) or for extra oomph a wand (I'm not sure knock was needed 50 times in 20 levels of adventure!)
wizard lock is worse in that a rogue cannot pick a wizard locked door. It either has to be knocked or battered down (although wizard lock makes that harder too. Further, it is permanent unless dispelled so very few resources wasted.
Both of these spells take a rogue and royally hose his niche (pathfinder at least made these spells arcana checks as opposed to auto success so there is that).
There are other offenders too of course: Comprehend languages and tongues, fly to mitigate/invalidate most terrain (which again are easily available on scrolls so no resource management issue) etc.
4e at least made most of these spells into rituals, giving them both a monetary and time cost (with the time cost being significant). Sure the mage can open the locked door, but it will cost 50 gold, take 10 minutes AND cost a healing surge (of which mages get the least of, I believe). The rogue on the other hand can do it in seconds, at no cost.
To answer the OP question:
It is a bug when it is too easy, steps on the toes of other classes, and in general all but replaces any skill use or creative thinking outside of "what spell do we use next" (which actually is not creative at all)
It is a feature when using the spell requires a trade off and a loss of efffectiveness (the wizard yelling ooga booga at a door for 10 minutes is a fine tradeoff to the effectiveness of the rogue) but can get the job done in a pinch.