Okafor is more skilled than Monroe and he's on a way better contract. Trading for Monroe has a huge opportunity cost in FA next year. Not so for Okafor.
I don't love Okafor, as he is bad on defense and one-way offensive centers aren't usually good players. But Okafor has a chance to be sufficiently great on offense to overcome his defensive woes. He's just extremely smooth and skilled everywhere inside 12 feet, both posting up and facing up. As his mid-range game becomes more reliable, I expect facing up will become a larger part of his game, as he has the quickness to blow by lots of centers, he's got all the spin moves and counters, an incredibly convincing pump fake, great touch on push shots, length to get his shot off over anyone, just the complete package.
I think that some people overlook Okafor's defensive deficiencies, and some people overlook his offensive awesomeness because of his mediocre efficiency. In reality he's an incredibly talented player with serious flaws. I wouldn't trade the #3 pick for him, but a lesser package could be pretty tempting.