It's worth noting that all the memory models have DS=SS, which makes sense for C (where you often take the address of a local variable - though nothing is stopping you from having a separate "data stack" for those) but is a silly restriction for some other languages.

I'm sure someone took advantage of this, but my knowledge is purely theoretical.

It's not necessarily true. Many drivers, TSRs and libraries (e.g. all Win16 DLLs) cannot assume that ds=ss. This makes C programming a bit more entertaining...

I never had SS=DS in Assembly. Used it for TSR for example.