It's not the party that gets 15% of the vote that concern me. It's the one that gets 2%, gets 5 seats and then effects significant shifts in national policy as other major parties solicit its support. I'm not American, but I shudder to think of the kind of fringe parties that would wield a disproportionate amount of power in the US if this were ever enacted. While "winner take all" election systems do encourage larger parties, they also encourage more inclusive parties that have broad platforms that encompass the numerous issues needed to rule a country. Proportional representation encourages single issue, or regional parties that.
The problem with the list approach, is that the most connected political insiders and hacks are the ones at the top of the list. If you don't vote the way the party leader instructs you, then come next election you'll find yourself way down the list. As a politician your main mechanism for ensuring re-election is to ensure you're near the top of your parties list. In Canada like most first past the post, parlimentary systems, it's possible for a party to win, while defeating the party's leader, or right hand man (this has happened several times).
Ultimately since it is politicians that vote in their respective houses or parliments, I would rather have the ability to vote for a person, rather than the abstract concept of the party