Man, I could make exactly the opposite argument. Rankings are weird in Dominion. The BM player who has no chance of winning will usually come second against two megaturn engine players (on a board where the megaturn engine is going to win), because he's steadily accumulating VP, even though it's not enough VP to win. Both engine players are playing correctly, but one of them is doing slightly better than the other. That's why first place is really what matters. If second place matters too, the strategy of the game is completely different (that could potentially be a good thing, but I don't think it is in this case).
I think you should play riskier with more players. In poker, you're a lot more willing to play hands like 7-6 suited in a pot with lots of other players, because even though it's unlikely to payoff, when it does, it will usually beat everyone else (contrasted with a hand like pocket Aces, which usually beats 1 or 2 players, but can get into trouble against lots of other players). Same thing in Dominion. If you're playing against a lot of other players, you have to take risks, because you need to get lucky to win. If you reward second, third, and fourth place, you just take strategies that steadily accumulate VP (i.e. big money). I think knowing when to take risks and what risks to take requires more skill than playing big money.
And, well, I'm not a fan of multi-player Dominion anyway. I was mostly talking about competitive "free-for-all" games in general (and like I said 5 million times, it depends on what game you're talking about). I think in most cases though, a "first infinitely better than second infinitely better than third etc." is best.