It used to be that only wineries and the occasional wine boutique store with a license to serve alcohol offered wine by the taste (typically a two-ounce pour) but with the growing interest in wines many wine bars have started offering more of their selection by the taste. It shouldn't even be a question that a wine bar would offer nearly every wine on the menu by the glass. But there are still many that don't see the point in offering wines by smaller than six-ounce pours.
A wine bar with a big selection should certainly offer a few by the taste, and in the reverse it's not surprising that bars with a limited menu offer none. The point of the cheaper, smaller pours is to get to know more wines over a similar amount of time and tracking over a theoretically identical inebriation range. It's hard to really sample four wines if into one's second glass one is becoming noticeably impaired. Additionally, it takes much more time if one is still intent on savoring each wine, never mind the potential cost. It's simply more sensible to order tasting pours in order to try out a real line up of flights. So a bar that only offers perhaps four whites and six reds may be looking at offering everything it has in one night if it offered everything by the pour. The return just isn't there.
However, for the wine fan, it matters what kind of a place a bar is versus what kind of an evening one hopes to have. Tasting is best done when one is not too distracted or else when accompanied by a like-minded oenophile. Certainly curious friends are invited to try out tasting, but the sort of evening given to hanging out, ranging conversation that never touches on the subject of wine and the sort of ambiance that favors drinking, but not exactly savoring and careful consideration, is probably best matched with full glasses of wine if not complete bottle service. Wine by the glass is great for a social night. Taste-sized pours are fantastic for discovering new wines. And carry a notebook for details of the experience. It confuses everyone and that's always entertaining.