15%!? In other words vote Labour or vote Tory, otherwise your vote counts for nothing (based on 2017 GE). 1.5% would clear out the niche parties!Indeed. And the fact UKIP, and the BNP before them, gained reaction precisely because of PR makes me wary.
I think FPTP is broken. I think PR is still the best alternative. But there are pitfalls. Really you'd need a floor, say 15% of the popular vote, to thin out the, er, 'niche' parties.