I suggested not using the sandbox because so many people have a hard time updating their sandbox settings to live settings.
The reality is that the eCart code works pretty well... If there is a problem it is usually with the settings. If you use sandbox you are really mostly testing to make sure the things we already have tested work properly. Your settings you can really only test by using the live mode.
I feel like by using sandbox you complicate the process and actually have a greater chance to introduce errors while moving from sandbox to live mode than all errors you might have fixed in sandbox mode.
Ironically I think sandbox is a good start for the more advanced coder who might be doing more customization and wouldn't have difficulty updating to live mode or debugging if something went wrong in that process.
To each their own, but in my opinion adding sandbox testing adds complexity that isn't necessary for the average user.