By outflanking with the deffkoptas on the charge they are str 7 with the buzzsaw and only 3 attacks, however you get the additional movement. However with Snikrot you are str 6 on the charge with 6 attacks and you can re-roll your misses, you also can come in from any side of the board you wish. Not to mention you need atleast 2-3 deffkopta's with a buzz saw to hit the tank with as much of a chance as snikrot costing way more than snikrot's point value.
Nothing in the ork codex can pull off what Snikrot can do, that is why he is there! Seriously... you gotta love Snikrot.