Short answer: Yes, of course they are stable. Long answer: Consider a car's wheels. They are mounted only on one side with five bolts to the suspension. You don't have to align your car wheels every day.

What is the advantage of a single-sided swingarm?

The main advantage of the single-sided version is ease of maintenance. A single-sided swingarm makes it much easier to remove the rear wheel, provided the bike has a centrestand or you have a paddock stand to hand.

What motorcycles have single-sided swingarms?

Ducati, KTM, MV Agusta, Kawasaki, and Honda all have motorcycles with super-trick single-sided swingarms.