The obvious prevalent use case is adding a business / work line via your company's VoIP provider to your personal handset (or a company one, for that matter). I

The obvious prevalent use case is adding a business / work line via your company's VoIP provider to your personal handset (or a company one, for that matter). I'm sure this is very common, and I have both done and supported it at multiple employers. Personally I have also in the past used VoIP accounts for cheap long distance, but I maintain a cellular service for reliability with local calls. Having this functionality integrate with the OS is much handier than the clunkiness of dealing with individual apps, and this idea of replaceable parts is a key advantage of Android in general. Emergency calls probably warrant special care and maybe more rigid handling, I will grant, but there's no reason not to support this in general.