The difference is a dump isn't often going to lead to an odd man break the other way unless someone completely blows their coverage.
Sometimes, but a few counterpoints:
1) If the dump-in isn't good enough (which happens more than just rarely), it can lead to an odd man rush as guys head to change and the puck comes back quickly.
2) One player carrying the puck in shouldn't result in an odd man rush either, unless he just loses it at center ice and both other forwards are already at the offensive blue line.
3) Many times that people complain when a player is losing the puck it's when it's in deep (I know I have complained about it in the past), which shouldn't lead to an odd man break any more than dump and chase.
4) Dump and chase more often than not also doesn't lead to a scoring chance vs. a player carrying it is.
I'm not absolving Necas of any sins here. He hasn't scored enough given expectations. He's not good enough defensively. He stays not the perimeter too much for my tastes. He disappears for stretches. He makes some questionable decisions at times, etc.. He has flaws and strengths like most players do and I don't think he's worth a huge contract right now.
I just think some of the narratives about him are overblown.