How about instead of doing away with partial credit, your reward is based on your participation scaled to the number of people that participated, or better, based on the time it took to down the mark.

Mark went down in 5 seconds and got 1-2 hits in? Get full credit.

Mark went down in 20 seconds and got 1-2 hit in? Get partial credit.

Of course, you also have to scale so that one cast from a blm or something is worth much more than one hit from a monk, considering cast time and etc.

I think that could strike a much better balance, since people wouldn't want to just leave after "tagging" the mark?