There is a difference between ReadyBoost and ReadyBoot.
On systems with more than 700MB of RAM, ReadyBoot uses data from 5 previous boots to create a plan for a boot-time memory cache. Similar to Windows XP prefetcher, it will try to preload files into RAM before they are needed. All memory used by ReadyBoot is automatically released 90 seconds after booting up, or immediately if another service needs it, so it doesn't have negative performance consequences.
In other words, on an SSD system, ReadyBoot may not improve boot times by a lot, but it will utilize your fast RAM for what it's good for: serving as a fast cache for the disk. And even the fastest SDDs are still slower than RAM memory - disabling it would still make your booting slightly slower.
ReadyBoost, on the other hand, is mainly related to utilizing flash memory (USB sticks) for the swap file. In this case, an SSD wins performance-wise, and there is no point in using a slower USB flash drive for caching, so Windows 7 automatically disables it.