It is not bad to use stemless wine glasses. Stemmed or stemless, juice glass, like some trattorias in Italy, you should drink wine from any other glass you like. But the reason many wine professionals don’t drink from stemless glassware is because the stem is there for a reason.
Stemmed wine glasses exist so that you are able to hold the glass by the stem, not the bulb, and therefore avoid warming the wine with your hand.
If you hold the glass by the bulb, whether it be stemmed or stemless, body heat can warm the wine, which causes it to lose some of the aromas and flavors that make that wine special. That may not be the biggest deal if you’re drinking a simple weeknight vino, but if the wine is something special, serving it in proper stemware is a good call.
