Yes, the service currently works only for those with Coinbase accounts. We do not promise anything else at this time. I'd appreciate if you suggest which services should be added next.
I believe exchanges provide a means for downloading your personal transaction history, typically in CSV format. Thus, it would be great to be able to import/upload transaction histories for US bittax calculations.
This may boost exposure of your service since people could experience your core service functionality without exposing their broker/exchange credentials.
EDIT: If you're going to ask for credentials as the only way to access your service then you may want to also consider other popular wallet services or exchanges such as Blockchain.info, Bitstamp, BTC-E and/or Bitfinex. You may already be working on integrating your service with these suggestions, but I didn't see that listed on your site beyond the "this is an MVP" section and the October 15, 2014 date listed in the FAQ.
The challenge for us is not to loose simplicity. It is very easy to end up having bloatware as the result of implementing all of the IRS guidelines. So, we will take a careful one step at a time.