There are a couple of issues with Runeglaive.
A) It feels bad to buy a unique passive that's worse than an item with another form of that unique passive without a major upside. Runeglaive lacks that upside. Junglers that need mana sustain can just buy Ranger's.
B) It's very slot inefficient. Magus+Ranger's is only really slot inefficient relative to morello's, and the magus enchant itself is actually -more- gold efficient than Morello's.
C) It's not going to improve AP jungler clear significantly enough to be worthwhile. The issue with most every unplayed AP jungler is the initial clear, which is long before this item is useful.
D) The junglers people are hailing this item as good for (Diana, Akali, etc.) are legitimately bad junglers that won't get played. Diana has no gank pressure pre-6 and Akali has no gank pressure at all. A different jungle item doesn't help this.
E) It's an overall nerf from Magus+Ranger's in stats.
F) I'm pretty sure the proc does less single-target damage after you combine than with just sheen in some cases. Not positive, but I think it's in that awkward area where the item has so little AP that you might actually get less dmg immediately after the combine.