"Can I have a taste of that beer, please?"
God, I hate that question on so, so many levels...
The First Taste
First, your initial sip of almost any beer is a very, very poor predictor of what you'll ultimately think of that beer.
If it's your first beer of the night (or day, if you're lucky), then your palate is reacting to the initial hit of alcohol, which (alcohol being what it is) is going to get numbed down pretty quickly.
Even if it isn't, flavor perception is an additive process. What tastes good might not taste good after more of it. A fingertip in the sugar bowl tastes good; eating spoonful after spoonful of it is disgusting. And something that tastes not-great initially might grow on you. But you won't know that, because you ordered a thimbleful of it and tried to make a prediction.
Second, as you drink more of something, the experience of it changes. Your palate will adapt to what it's tasting, and things that are unpleasant at first can fade away or mutate into something pleasant or even transcendental.
That 120-IBU monster might seem too intense for you if you just drink one ounce. But eight or 16 ounces later your palate has gone a bit deaf to it the bitterness and instead you may be tasting something very, very different thanks to that high IBU burn-in.
And we're not only tasting - we're feeling. That sharp sourness will seem much less so after a few sips, but the puckering tightness will still be there, and that might be something that changes your evaluation.
You'll never experience that, though, because you took one sip and moved on.
Third, beer is volatile. It changes. Some flavors will come right out of solution and dissipate in seconds - if you reject the beer because your little taster had that flavor and you didn't like it, you're walking out of a movie five minutes into it.
What if the beer is overcarbonated? As it sits and approaches the "right" carbonation level, it will change, and its flavor will change. You'll never know, because you passed after two sips or your sample.
Use the Right Tools
Your taster probably came in a completely different glass than your beer would have. You probably aren't getting much of a sense of the aroma. Any fault from a not-beer-clean glass is hugely magnified because of the surface-area-to-beer ratio in that tiny glass. CO2 is being released differently on that lip.
In other words, you're not even tasting the beer you'd be getting by the pint.
Pouring These Sucks
It sucks for the bartender, but it also sucks for you. Beer service off of a tap into a 2-ounce glass isn't the same as the same pour into a pint. The system isn't designed for tiny pours.
And it's a pain in the ass for the bartender - have a heart.
Drink for the experience
I don't order a sample of anything. What's the worst case scenario? I drink it fast or give it away?
When I order a beer, I want the full experience. I want several sips of it. I want steadily declining carbonation. I want all of the palate sensations. I want to work through what I might consider "off-flavors" and maybe come to appreciate or understand or learn to ignore them.
And I want to be a considerate patron, because a) I used to tend bar, and b) it gets me my next beer faster because they know I'm not going to ask them to pour three or four or seventeen tiny samples, and c) I'm only human and want people to like me.
So please, don't order a taste of anything.
Keep it simple.