I could see people using this to get up and running quickly, but then flushing out their system to integrate directly with carriers once size dictates.

Example: DailyCred lets you drop in authentication on your site, but when you get big enough (or have the time resources), you can dump out the bcrypted password hashes into your own built-out auth system.

MVP/Prototype->Iterate->Grow. These services that are reducing pain points let you get the first parts done fast, so you can get to the growth stage faster.