Whilst it is often true that for many provinces a market wont break even for a decade or two it does provide other benefits, such as population growth which for low growth provinces can be useful.

Upgrades also have benefits in that assassins produced in cities with better trade buildings are on average higher skilled than those produced in cities with out trade upgrades.

As someone whose playing style is of a builder rather than blitzer I find markets etc useful though usually only for coastal cities.