Just to complete the list of online banking systems before we go back on topic:
Here in Switzerland you need a token for the online banking of every bank. It's needed for the login and often for transactions. There are different options (a device that generates a OTP code with the card, an USB token, a device that generates the TAN from an image and of course different iOS / Android Apps) depending on your bank.
Luckily, there are still "non-mobile-device" options, but sadly the trend is pointing towards "mobile first" and with that iOS / Android lock-in.
BankID seems to be a good example which already discriminates against those without iOS / Android. While I'm a big fan of open implementations, I think it's very unlikely that this iOS / Android lock-in will stop any time soon (but that is a topic for another discussion).

So offering support for all those essential Apps without having to use a spying system is a huge feature from my point of view.

Regarding the battery consumption, it's really hard to tell. All you do in emulators is basically mapping the System Calls of the emulated system (Android) to their equivalents of the host system (SFOS). This should basically have no big impact, but it greatly depends on the guest system and if it requires specific services to run constantly in the background.