A seamless or bonded balaclava is absolutely worth it if you ski with a helmet and spend full days on the mountain. Traditional sewn seams can press into your forehead and temples under helmet pads, creating annoying pressure points that get worse as the day goes on. Seamless construction eliminates that issue entirely since there's nothing to dig into your skin.
This style is especially great if you have sensitive skin or get irritated by stitching rubbing against your face in cold, dry conditions. The smooth, uninterrupted feel also helps your balaclava sit cleanly under goggles without creating gaps or uneven spots that let cold air in.
The one tradeoff to know about: seamless construction can be slightly less durable at stress points compared to flatlock seams. If you're tough on gear or frequently pull your balaclava on and off, keep an eye on stretching over time. But for most resort and backcountry skiers prioritizing all-day comfort under a helmet, seamless is hard to beat.