One could argue that the speed limit is a lot more of
an arbitrary number than an aircraft's gross weight figures.

Like the max gross on the Warrior 161 that can be changed by putting a
sticker in the POH? Or the max gross in that Cessna that can be changed
by limiting the flap travel? Or the max gross on the PA-32-260 which is
the same as that on the PA-32-300, despite having a lot less power for
take-offs in marginal conditions? Once you start deciding for yourself
what is acceptable and was isn't in terms of speed, and ignoring the
"experts" who set the limits, you're in the same position as busting
max gross. You're thinking for yourself, and ignoring the rules. Thus,
if one is likely to make you an across-the-board rule breaker, so is
the other. There are good arguments against over weight operation, but
this slippery-slope argument isn't one of them.

Also, let's do away with this by-the-book argument. None of us fly
light aircraft by the book. We don't take-off from fields that are
*exactly* what is stated as required by the book and we don't land into
fields that are *exactly* what the book says we need to stop. We add a
safety margin that we are comfortable with, and that is based upon our
own experience of the aircraft that we've built-up over time. The book
figures are, as we always reminded, with a new aircraft with a pilot
who is probably a damn sight better than we are. So, since our aircraft
aren't new, and since we're not that good, we might as well say that on
every take-off, we're a test pilot, since we're operating in conditions
that aren't documented in the POH.

The fact is that over-weight operation is not particularly dangerous
unless you're out of balance or in marginal conditions re the take-off
in the first place. The structural effects aren't going to be a
problem, and the stall speed effects aren't going to be a problem. The
failure mode that matters is failing to get out of ground effect, or
failing to get into the air at all. And that is something that can
happen whether or not you stick to max gross, and that you have to use
your judgement to decide upon based upon your knowledge of the airplane
and what you're comfortable with. So while it isn't legal, it isn't
particularly dangerous, and it is far more common than most people
would admit.