As a gamer i play all of it. I play the character build sometimes more or the RP sometimes more, and it is arbitrary from play to play as to which i may be doing.
When i have a good specific character play i want, then the mechanics of build are mostly ignored in favor of the RP POV dictating my build mechanics choices.
DocBot is a DocBot. No DocBot is equipped to do any harm, they only render support aid. SO DocBot has only 3 attack powers, ones i as a player was not allowed to mechanically forgo.
So i have the starting secondary blast power, the background attack power every one starts with, and brawl.
However none off those powers get any upgrades and none of them are slotted on my bars. only DocBot powers are ever used by the DocBot.
At other times I have a specific mechanical build in mind. A class/set of powers i want to run because i find them mechanically interesting.
in these cases i probably still give some RP to the character, but in this case the RP is derived from the character build, rather than i form a character in my mind that informed power choices based on their personality.
kind of like reverse engineer the RP.
Common RP is: we imagine a personality and then that informs class/skill choices.
but you can actually start with the power/skill set and imagine the personality that made those choices.
So for me it depends on what's been of interest in my brain of late.
more mechanics interests is to build the toon and determine the RP from that.
more story telling interests is to imagine the character and build the toon from that.
But yeah when it comes down to it, if it is the RP that is the focus of the fun, then i can't see the "objectively irrational mechanics" choices being made, these are the rational choices from the character POV.