You can carry ski boots in any bag big enough. The question is what happens to everything else in it.
What a boot bag adds
- A separate boot compartment. Wet boots and dry clothes in one pocket is how you end up in damp base layers. Boot bags keep them apart, often with a water-resistant lining.
- Ventilation. Liners dry faster, and the bag smells better in March.
- Structure for heavy loads. A pair of boots is heavy and lumpy. Padded straps and a back panel spread the weight.
- Helmet storage. Many boot bags have a helmet canopy or a dedicated section, so the helmet isn't clipped to the outside.
When a regular backpack is fine
- You ski close to home and boot up at the car
- Your boots dry in a ski locker at the mountain
- You only ever carry boots a few steps
Boot bags for flying vs. driving
The two kinds of trips need different bags.
Flying: size limits matter most. A carry-on boot bag such as the Weekend Warrior or the wheeled XL is built to the dimensions airlines accept, and adds a laptop sleeve.
Driving: capacity matters most. Diamant's Tank is a 70L day bag with a pull-out mat for changing into boots in the parking lot, so you aren't standing in slush in your socks.
The short answer
If you fly to ski, or change in parking lots, a boot bag is worth it. If you ski from a locker, a regular bag will do.