Just a stab-in-the-dark here but no device actually overwrites deleted data. They simply flag the formerly allocated space as free, a privacy hole in some cases.
With flash, wear leveling prevents direct sector reuse thus limiting wear in any one area. Still I think cost is too high & lifetime to low at the moment to justify the gains. Cost has prevented me from trying SSD out which is likely a good thing for I am sure it's faster.
