Well, among other things, it depends on what the pill does.

Actually, I would argue that it's not good regardless of what the pill does.

then I can't see any benefit at all

That's 'cause you're looking at it too narrowly. Is there a benefit for the doctor who sees the guy and who gets paid for it? Sure is. Is there benefit for the company which makes the pill and sells it for a nice profit? Sure is.

if being prescribed a pill that ... inevitably subjects me to social attack

What subjects you to social attack is having been diagnosed with a mental illness.

There's all kinds of things I can do

It's not what you can do -- it's what can be done to you.

if the pill actually does make me less grumpy without too many side-effects, that's a pretty good sign that there's a biological (or biochemical) cause for grumpiness.

Not necessarily. A common description of the effects of SSRI anti-depressants on some people is that they make you feel completely indifferent inside. Sure, you don't want to kill yourself any more, but you don't want ANYTHING. I am pretty sure that if you react to anti-depressants this way, they will also make you less grumpy. That doesn't mean grumpiness has a biochemical cause.