2-way stretch base layers stretch in one direction—usually horizontally around your body—and then recover their shape. For most resort skiing and recreational riding, this is plenty of mobility. You'll be comfortable carving turns, riding lifts, and making normal movements on the slopes.
Where 2-way stretch can feel limiting is during extreme range-of-motion moves like aggressive park riding, hiking to backcountry lines, or deep mogul runs that demand reaching and twisting in every direction. If that's your style, 4-way stretch is worth the upgrade.
The good news is that 2-way stretch garments still include elastane for shape retention, so they won't bag out by the end of the day like non-stretch base layers can. For the majority of skiers and snowboarders cruising the mountain, 2-way stretch hits the sweet spot between mobility and value.