Some argue they look better, and if you're an endurance racer needing to swap wheels in a flash, then they do have an advantage.
However, that comes at a cost: weight. It's pretty logical that a single sided swingarm has to be heavily braced internally to be as strong as a twin-sided swingarm.
Easy to prove it to yourself:
With both hands, hold a wooden spoon (or wheel spindle!) by one end - holding it horizontal. Now have someone press down on the spoon / spindle, and see how easy it twists / moves downwards.
Now hold both ends of the spoon / spindle, and have them do the same thing again. Much less movement, even though its the same two arms / hands holding the spoon.
Whether that weight makes any difference to you is a matter of opinion... Some like the looks and sod the weight, some think a two stroke 250 is meant to be as light as possible, so why make it worse?
How many MotoGP bikes are running single sided arms?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball