I think the difference is that in the case of sharing a wall with a neighbor you have a) extra noise leakage that you wouldn’t if you had that extra layer, b) s

I think the difference is that in the case of sharing a wall with a neighbor you have a) extra noise leakage that you wouldn’t if you had that extra layer, b) shared infrastructure, which makes things a little harder if you need to make changes/have a plumbing issue/etc. Also the roof is continuous between townhomes, so when it comes time to replace that you have to negotiate with the neighbor.