All melee champs should get Mr per lvl or be balanced to get it.

I feel it is very poor design for these melee champs to be lacking in stats in order to balance their overloaded kits.

kassadin - he gets free Magic damage reduction from his passive, but currently his kit is very fighter based and a bit overnerfed

Maokai - Is a Tank but still does not receive and Mr in stats

Poppy - is excused since her passive makes resistances less effective (unless it's a DOT)

shen - only has sustain and a lackluster shield to avoid damage

Singed - Has to absorb every CC known to man in order to fling anybody but the tank into his team and deserves this without compensation

Yasuo - Should get his double crit moved to his Q only and his windwall width or duration should scale with his current flow and his base damage on q reduced to 10/20/30/40/50 and crit penalty removed or scale with lvl like shaco

and get MR per lvl to compensate

another good solution is to stop releasing champs with overloaded or broken kits so that they don't need to be balanced this way. just my opinion though.