ActiveMerchant's entire purpose is about abstracting all the different payment gateways into one API. If you know for sure you're going to use Stripe, or if you want to use Stripe specific features (of which there are quite a lot worth using) ActiveMerchant is less valuable to you.
It's also worth noting that ActiveMerchant hails from a time long before Stripe or even Braintree, and so it supports gateways that are quite a bit more complex. (disclosure: I wrote the ActiveMerchant Stripe support).
It's also worth noting that ActiveMerchant hails from a time long before Stripe or even Braintree, and so it supports gateways that are quite a bit more complex. (disclosure: I wrote the ActiveMerchant Stripe support).