Wish I had a buck for everybody who tried to pass the buck by claiming to "lack capacity."
It may seem foolish to portray oneself as a can't-do person, but like any buzzphrase, I think this one has spread because it captures a complex truth. We all understand and have incorporated into our lives the notion that scarcity boosts value. Unfortunately many of us seem to have gone a bit overboard and concluded that if we, ourselves, are scarce, then our value will rise.
The truth in this is reflected in the way one feels one has won a lottery prize after managing to "get some time" with an overbooked executive or politician. The frequent observation that chief executives are godlike is apt: they spend a lot of time in the sky, walk among us only rarely, and use their power to reward or punish in unpredictable ways. Every organization is entitled to a small pantheon of these creatures.
It's fine to dress like the boss, but acting like the boss before actually becoming the boss might not be the sharpest thing to do. To say yes to everything is to invite abuse, but to say no to everything is to become reviled. Regardless of the business you're in, there has to be service involved.
Besides, to claim lack of capacity isn't the same as claiming lack of intelligence or ability; it's to cleverly suggest that one is a bit of a victim, that if only circumstances were a little different, one would be thriving and would be able to help with whatever the request was. As it is, well -- wish I could help, but it's just not possible. In this way it's possible to both claim and reject accountability at the same moment.
I would rather hear "I'll do my best" or even "hell, no" than "I lack capacity."