[stephencanon]

Biggest potential weakness seems like the ability for the merging party to strategically collapse districts such that two legislators from the opposing party are made to reside in the same district, creating a new district with no incumbent. Not all legislatures require that representatives live in the district, and there would be real trade-offs to doing so, but it would be a pretty powerful hammer to wield against up-and-coming opposition candidates.

It doesn't matter at the abstraction of proportional representation, but it potentially matters quite a bit when you get into the nitty-gritty of actual elections.

[maxpalmer]

You're right that this method doesn't protect incumbents. However, protecting incumbency and avoiding open-seat elections isn't necessarily a bad thing, and could increase electoral competition in some places. Some states don't allow incumbency to be taken into account when redistricting already.