Most chargers with an internal PSU cannot be used to the max specs. I have an old Duratrax charger which can charge 3A on its own PSU and 5A on an external PSU.

But yes, if you have AC chargers which can be used to the max (or within your needs) then there is no need for an external PSU. But an external PSU could be very handy for the supply of other equipment like tire warmers, Proxxon 12v multi tool, 12v LED light etc.