I agree that making yourself crazy with 100% adherence can just drive up
stress levels.  Who needs that.  I don't follow it 100%, but I should say
what I mean by that.  I mean I don't make pemmican and hunt my food with a
stick, I do eat a sprinkling of New World and nightshade--to as best as I
can tell my own tolerance--and I have the occasional drink, even.  While I
do try to eat as organically as possible, economics and logistics have me
making tradeoffs there, depending on the day and season.  I try to eat
seasonally, but do eat my fruits from Chile in the winter.  If it's a choice
one night, after a long day in which sudden changes in my schedule kept me
out late and I'm just too exhausted (with my disability) to cook, between
eating a non-grassfed takeout piece of meat, or going really off and getting
a takeout junkfood meal, I will definitely get the nongrassfed steak or
piece of chicken or salmon, better bet.
Anyway, those are some ways I'm not always strict.  However, I try to stay
relatively ON paleo in that I try to avoid grains, a lot of dairy (some
quality butter does get used a little), and nitrates and processing
additives, and commerical sugars.  The argument for staying on it as much as
possible, for ME, is that I'm already borderline type II, so my sugar levels
have a very small 'fudge' window; I can't get away with much and not feel
badly/risk my health.  Also, again for me, I find that the addictive
properties of carbs like grains and sugars have an immediate effect.  In
other words, I had to fight a weeks long-battle against my body and it's
cravings to get off them the first time(s).   I hate reintroducing them and
then have to fight more battles to re-get off them again.  Who needs all
those mini-withdrawls?  They're no fun, and you risk NOT getting back on the
wagon after one of them.  For me, with my  health, that potential
downside/risk is so awful that I really really monitor my cheats.  I plan
them out, and never let myself have an impulsive cheat.  Even with planning,
I know I will suffer later.  I actually make a pact with myself that I do
not try to mediate that suffering--ie, if the next mornning my sinues are
clogged and my fingers already swollen and arthritic (it only takes one
serious dollop of wheat/grains to do this!) I don't take aspirin or
anything.  I feel the pain.  It's an important and potent reminder of why I
want to fix my paleo breakfast and get on with it.

So, my arguments against going off in sum are:

a) have reintroduced addictive substances and must go thru withdrawl again
b) lose the nice rolling state of non-craving have built up
c) risk going off plan completely and wrecking health
d) mentally feel down about it

This helps me, to write this now, as my 48th birthday is tomorrow.  Yes, I
have cheats planned:  a) a small cup of my fav ice cream and b) a margarita
with my paleo mexican dinner (ie deconstructed (no tortillas) chicken and
steak fajitas with homemade guac).

best,
Ro