The way I see it:
First you have people who bought multiple game registration codes for the main purpose of cheating the system and participate in competitions multiple times.I doubt these persons would have linked their POL accounts to the same SE account, out of fear/paranoia of being detected by SE.

Then you have power-gamers who have multiple POL accounts in order to play several characters at once. I know I am just making guesses now, but these players do not strike me as the kind of players who are heavily into creative competitions, and even if they were, just handing in two contributions doesn't mean they'll win with any of them, as they also need to actually be good.

Thirdly, you have people who combine their POL accounts for management purposes, where several people in the same household just want one single place to pay for their expenses.

I don't think there is a very large overlap between these groups, so I don't think it would have a very big negative impact on competitions if they allowed each POL account to participate, rather than each SE account.

In either cases, cheaters will be able to cheat if they really want to. The difference in cheating-difficulty is very small no matter which system they go for.