Yeah that's a good point. Although, we don't make it very clear, you can connect your Google account and then opt to only get changes from your Basecamp account. But yes, you would always need to sign in using your Google account. Thanks a lot for the feedback.
Perhaps create somekind of call to action that's more like this: 'Connect your Basecamp account', they end up by creating a normal account and then after that the first step is to connect their Basecamp account. This gives you a way to learn what webservice people would connect first. But hey, enough ideas. It's all about execution. Good luck!