If you try to base it off of mechanics, people will just flock to the mechanic that gives them the best chance at gear and ignore all else. For example, say you had a better chance of getting loot from LFR Will of the Emperor based on the number of Opportunistic Strikes you made. No one would want to focus on adds lest they reduce their chances of getting gear from that boss. Plus this method doesn't account for latency spikes which is something you can't control.
If you base it off who could benefit from an ilvl upgrade in a slot, you eliminate a big reason for people running normal/heroic raids to queue as the Valor that's gotten for a random LFR is slower than just running dungeons or scenarios.