There is also an issue in that MPs votes mean different things at different times.

For whipped votes, they are party votes and there could be an argument about aligning them better along party-vote-share lines.

But for unwhipped votes, or where an MP rebels against their party, they are potentially representing all of their constituents, regardless of party affiliation. As all constituencies should be a similar size (and may be, after the forthcoming political reforms), each MP's vote should have the same value.