Can you explain why that wouldn't lead to the introduction of parties like Tories2, Tories3, ..., Tories100? One could allocate 1/#votes "points" to each party voted for, but that gets you back to strategic voting.
That was my first idea, but the comment I'm replying to included
> This would remove a lot of tactical debate if you're vasting your vote on candidates/parties who don't have huge chances for success.
which doesn't work if you give each party 1/#number_of_parties_voted_for votes (voting for CoolGuy gives your vote on EstablishedAndNotTooBad less weight, after all.)
Creating a party and running it in an election is a tad more onerous than creating a Reddit account. You can't create 100 parties just to try to game the lottery.
