For suits with pockets I see the need for a skin but suits without pockets is it all marketing?
I can't speak for other brands, but I can say that for us (Aqua Sphere), it is not just marketing. Limits are provided by governing body's though.
There is a lot of development that goes in to a swim skin. Number one is developing a textile fabric that has a medium-ish level of compression but also produces a fast surface with very minimal drag. Newer suits are also adding more paneling that provide more compression benefit that aids you while swimming.
Monty mentioned fitting in the tightest suit possible. This is very important to achieve the greatest benefit. The tightest suit that doesn't restrict your breathing or range of motion.
If you look at swimmers and tech suits as an example, they most definitely work. Triathlon swim skins are using very similar technology. Tech suits that swimmers wear take a considerable amount of time to put on because they are so tight. A swim skin is designed to be fast in the water, but still be easy to take off. The textiles used are similar though.
For the benefit, it will vary by each athlete, and will not be as big of a benefit as a wetsuit for most people. That said, there is generally still a measurable benefit.
I'm biased, but I have done a fair amount of testing myself, and I do see the benefit.