A pack with a reservoir sleeve but no insulated hose routing works fine for resort skiing and milder backcountry days. You still get the convenience of a hydration bladder and hands-free drinking through the tube—just without the extra protection around the hose itself.
The tradeoff is that in sub-zero temperatures, the exposed hose and bite valve are more likely to freeze, which can cut off your water supply when you need it most. If you're mostly riding in springlike conditions or warmer resort days, this usually isn't a big issue.
One easy fix: always blow air back into the hose after each sip. This clears water from the tube and bite valve, which goes a long way toward preventing freeze-up even without insulation. But if you regularly tour in deep cold, consider stepping up to a pack with insulated hose routing instead.