I assume it's a well thought through and properly risk-assesed security/convenience tradeoff. Handling encrypted files is much less frequent than handling encrypted email - and putting an airgap between the internet and your email is likely to cause more grief than the security improvement it creates.
I've got the seeds of an idea which has been kicking round my head for a few weeks now - a rasbperrypi (or similar) with GPG installed on it, which is connected to my main computer as a usb device (possibly impersonating a usb keyboard). The 'pi could be sent encrypted data over the usb/serial connection, and send back the plaintext. The 'pi would have no network connection – reducing the attack surface for someone trying to extract my private key remotely to some exploit that'd work over a tightly constrained serial connection. Sort of like a RSA SecureID on steroids - here's a device with a "cryptographically secure secret", but instead of just displaying TOTP tokens, you can feed it encrypted data and have it send back cleartext (optionally with a keypad and PIN/passcode required, but it's not "secure" against physical access, so I'm probably not going to try and implement that...).
That definitely fits with the 'hacker ethos' and all, but why not just use a smartcard with a Class III reader (i.e. dedicated pinpad and display on the reader itself).
Support is already integrated with GnuPG, they are specifically designed to prevent key material leaking, and they have some other nice properties (like self-destruction after three incorrect admin PIN attempts).
Mostly because I've got a pair of RspberryPis – and I'm doing this mostly out of curiosity and learning (and a little bit of "sticking it to 'the man'"…).
If you have to store encrypted credit card data, that's the recommended way of keeping it safe. Your Pi is analogous to hardware encryption devices that have been available for some time.
I've got the seeds of an idea which has been kicking round my head for a few weeks now - a rasbperrypi (or similar) with GPG installed on it, which is connected to my main computer as a usb device (possibly impersonating a usb keyboard). The 'pi could be sent encrypted data over the usb/serial connection, and send back the plaintext. The 'pi would have no network connection – reducing the attack surface for someone trying to extract my private key remotely to some exploit that'd work over a tightly constrained serial connection. Sort of like a RSA SecureID on steroids - here's a device with a "cryptographically secure secret", but instead of just displaying TOTP tokens, you can feed it encrypted data and have it send back cleartext (optionally with a keypad and PIN/passcode required, but it's not "secure" against physical access, so I'm probably not going to try and implement that...).