The reason I'd rather keep Matri is exactly because he's, at least somewhat, more comparable to Llorente than Quag. If we hold true that two different positions are to be filled on the field, then we must hold true that at least 2 players suited for each of those positions should be in the squad.
Regardless of intermittent experiments, Quag is not a spearhead. When he does his best work he works in the space created by such a spearhead. We already have two players better suited for this role in Vucinic and Giovinco.
Now, if a better alternative to Matri as back up for that spearhead position taken by Llorente should present itself, by all means, 'so long, Matri.' Until then, I think he will stay and rightly so.