ActiveMerchant is just an abstraction layer around various gateways. It's goal was never to provide a full-stack solution, but to simply making working with numerous gateways easier because you have a common interface for working between all of them.
As for subscriptions, that's a feature that could be added but many of the Gateways that ActiveMerchant supports don't have subscriptions baked into them (compared to say Stripe).
You could of course have your application store the credit card information and manage the charges yourself using ActiveMerchant, but that opens a bunch of PCI compliance and such.
If you just need to accept money and aren't already bound to a specific merchant account, then plain ole Stripe or Payola are a better option.
As for subscriptions, that's a feature that could be added but many of the Gateways that ActiveMerchant supports don't have subscriptions baked into them (compared to say Stripe).
You could of course have your application store the credit card information and manage the charges yourself using ActiveMerchant, but that opens a bunch of PCI compliance and such.
If you just need to accept money and aren't already bound to a specific merchant account, then plain ole Stripe or Payola are a better option.