It would change the dynamic, but the issue is the fact that the presidency is "winner takes all." In a multiparty legislature, they parties compete at the polls

It would change the dynamic, but the issue is the fact that the presidency is "winner takes all." In a multiparty legislature, they parties compete at the polls, and THEN they form a coalition. The stakes for the presidency are such that it encourages coalition building BEFORE the polls, to produce a candidate with an expected majority. You can see the dynamic during some of the less polarized political cycles of the past. The parties and candidate had to build coalitions of "interests" groups that weren't necessarily pre-committed to one party or another. I could imagine a transferable vote system looking getting us back to that "coalition" state where the parties platforms changed based on the dynamic coalitions.