3/4 length base layers are actually a favorite among serious skiers and boot fitters because they end mid-calf, right above your ski boot line. This eliminates fabric bunching inside the boot, which can cause pressure points, discomfort, and even restrict circulation on long days. A smooth, bulk-free fit inside your boots makes a bigger difference than you might think.
As for warmth, your ski socks cover the small gap between the base layer hem and the top of your boot, so your lower calf isn't actually exposed to the cold. As long as you're wearing proper-height ski socks, you won't notice any loss of insulation.
3/4 length is an especially smart choice if you've ever felt tightness or numbness in your feet and lower legs while riding, or if you're working with a boot fitter on a precise fit. If you run really cold and prefer the security of full coverage, full-length bottoms still work fine—just make sure they fit smoothly or have stirrups to keep them in place.