[{quoted}](name=Devi Freak Mint,realm=NA,application-id=3ErqAdtq,discussion-id=UxjrHod7,comment-id=0000,timestamp=2015-02-11T18:35:48.543+0000)
Very few champions can use both armor and magic pen effectively.
Every champion is likely to take both physical and magic damage.
In most cases, Hybrid Pen using casters are trading very little magic damage for a very small small increase to their Basic Attack damage against enemy champions (and cannon creeps).
If we followed your suggestion, we'd be trading some defensive stats to get more of the same stats (not much of a trade is it?).
More importantly, you're barely even trading any stats with these numbers. Compare to ar/mr quints:
AR: 4.26
MR: 3.00
You could make this a balanced option- you'd just have to give up more of each stat. Say, give the quint 2.25AR/1.9MR, That way, the quint is marginally better in terms of gold value than AR or MR, but not as good against a specific type of damage, making laning vs a single dmg dealer worse.
All of that said, there's really no reason to make such a quint. If we start making hybrid everything, where do we stop? Hybrid Crit damage/hp% runes?