Interesting, but seems more like a one-of or subtheme than an actual mechanic. Also the basic effect is pretty black/blue. Is this supposed to be Gruul? The name fits, but the mechanic is a very poor color pie match.

On further thought I can see three strikes against this mechanic:

1) It's a hate mechanic. This means it is highly situational, and its very presence requires far more +1/+1 counters than would otherwise be present. Also you simply cannot draft a "pilfer" deck.

2) It's a punisher mechanic, and one which gives more control to the opponent than almost any other before it. And it's not helped by the fact neither black nor red can make creatures block as part of their pie (it's the opposite, in fact!).

3) It's just not very Rakdos. Mechanically, the guild has usually focused on recklessness. Stealing stuff feels more Gruul, Dimir, or even Orzhiv than Rakdos.

The front page says Goblins as a tribe are blue-red for this.

The other points stand though, and blue-red doesn't seem like the natural place for this to go. I assume this is a top-down to pay tribute to the Boggart flavor of taking stuff.

This mechanic has very many changes that could be applied to make it less problematic, but w/o info on what the design parameters are, it's hard to pick the best alternative.