Depends on the application, really. I can give you an example of where I've personally considered using this (ended up going with an alternative, however): my startup's product is a hotel front desk system and we have to encrypt room keys to work in the locks; pushing the crypto off onto an external device would make it considerably more difficult to pirate the software, as you'd have to reverse the algorithm and reimplement it in software. In the end, it didn't make sense for us, but it would've been pretty solid, as the odds of you having two identical cards is monumentally slim (and would only even be possible every couple of years).