An argument could be made that "easy" encounters are unnecessary filler fights and that every encounter in a game should be challenging and rewarding.
Being able to breeze through easy fights is fun once or twice - but becomes boring at some point (at least for me). So for me (who likes several RTwP games and has no clear preference) this is not a convincing argument.
TB combat does indeed take more time if you insist to use too many combatants. Some encounters are fun with a ton of enemies - and if it's just to give you the feeling of a chaotic, "swarmy" combat (hello wichts in PoE) and here RTwP has an advantage. But it is easier to design and balance challenging yet fair encounters with TB.
I think RTwP caters more to an audience that also likes action RPGs games but isn't overly fond of chess (don't take this literally) while TB is more of a puzzle. There ae exceptions of course (FTL vs. Into the Breach - one is RTwP, the other TB - both quite puzzle-ish). I like both approaches.