Will a 2010 MacBook Pro without SATA III benefit from a 6 Gbps SSD?

Only high-end SSDs with decent Sandforce controllers are able to enjoy the full 6Gbps speed (= 600MB/s). Older generations can only provide R/W speed in the 200-300MB/s range which is within the capability of SATA 2.0 (300MB/s).

All SATA versions are currently downwards compatible, so a SATA 3.0 disk can work with a SATA 2.0 port but the peak (external) transfer speed is limited by the port in this case. Thus you will not have the full benefits of the new interface.

However, the new generation of controllers may bring other performance improvements over their SATA 2 counterparts, so they are still worth buying. But if you're not planning to get a new computer any time soon, you can get one with slower access speeds, e.g. around 250-300MB/s. (Flash chips are pure silicon so they follow Moore's Law, so by the time you get a new computer, the prices of these SSDs must have dropped so significantly that it's not worth buying the expensive ones now.)