Dual compression—having both side and vertical straps—isn't overkill on a 35L pack, especially for backcountry use. Throughout a touring day, your load shrinks as you shed layers, eat food, and drink water. Both strap types let you cinch down the pack as it empties, keeping your load tight against your back so it doesn't shift and throw off your balance on the descent.
Side straps handle horizontal stabilization, while vertical straps compress the pack's height—which is particularly handy if you carry skis diagonally. Together, they give you the most control over how your pack rides, no matter how full or empty it is.
The trade-off is a bit more weight and more straps to manage. If you prefer simplicity and mostly carry consistent loads, side compression alone may be fine. But for variable backcountry loads in a bigger pack, dual compression is worth it.