Seamonkey is also convenient is you want to run another Mozilla browser alongside Firefox and not have to take any measures to keep the profiles separate. So it adds one to the number of browsers you can just install and run with no special setup and thereby split some of the advertiser & Facebook tracking that is so annoying.
Seamonkey and Thunderbird also keep the Mozilla team somewhat coherent in developing the common codebase, though increasingly build issues are wasting a lot of time for those two now unpaid projects. Mozilla has three projects it supports with paid developers: Firefox, the Firefox OS and Firefox Mobile. It dropped Thunderbird recently from that group and it's not clear how the TB team is going to handle rapid release vs. extended service release. Lots of tricky work for unpaid developers to keep up with an intricate codebase continually special cased for the three paid products, and to match Chrome innovations.
Seems to me Seamonkey developers are the ones most concerned with making current features work predictably for users.