Why Not Keep Magus and Runeglaive?

Why should Runeglaive replace Magus? I love Rineglaive as it looks like a fun item for Diana and several other AP junglers but it Nerfs Fiddlesticks, AP Amumu, Karthus Jungle, and probably more that I can't think of. Yes I am aware that the only viable jungler I listed was Fiddle but still not having Magus limits the amount of experimentation that can go on in the jungle and to me the jungle is one of the most versatile roles due to the lower chance of punishment from the enemy since they are likely using their time ganking or farming their own jungle rather than trying to counter jungle the jungle Karth or whatever the weakness of the jungler in question is. What do you guys think? I am eager to hear if any of you know of a benefit to removing Magus instead of just adding in Runeglaive. I would much rather have dual AP items.

I am an unranked player and my skill is probably silver or bronze so needless to say you probably could educate me on what I'm missing