A good argument for both switchblades and gravity knives has been that they are needed for those who only have the use of one hand, and even that one hand might be compromised, such as by arthritis.
But knife makers seem to be missing a market segment by not specifically designing switchblades and gravity knives to be of particular utility to the handicapped. Instead their focus is on fixed blade “adaptive knives”, mostly for kitchen use.
http://products.disabled-world.com/product_list.php?category=659
Yet knives remain a primary tool no matter where you are. To date the handicapped are limited to switchblades and gravity knives that are more stylish than practical. Many use lower quality steel and have fairly dull blades.
There are many assisted opening knives legal in the United States.
Here is a great example: Kershaw Scallion Folding Knife with SpeedSafe
It is not a switchblade - a switchblade’s blade loads a spring and is kept in place with a latch.
These knives have a torsion bar which keeps the blade in the closed position and also helps it open and lock - no latching mechanism whatsoever.
Excellent knives and extremely easy to open (automatically) and close with one hand.