Slime has portability rules that are of specific interest to Seaside development itself.
Developers of Seaside applications are commonly not interested in making sure their Seaside application code is portable across different dialects. However, there are Slime rules in Seaside that are of specific interest for Seaside developers.

Right now, developer should enable/disable the rules themselves.
It makes more sense to disable the Seaside development rules by default and enable them when loading for Seaside development.