Well, the beauty about that service is that it sends you the binaries for all supported platforms. I understand this is better for HTML "developers", since they don't have to deal with several different SDK installations, IDEs, etc...
As for iOS apps, I don't think the IPA file has to have the developer ID as you said, since this PhoneGap service says it will send the IPA file for you and then you'll have to proceed and sign it. What I know about it is what I read on their website, so it might be not completely true...
What I like the idea is that you can have multiple binaries created at once. It saves a lot of time, and since the Tier 1 is very platform agnostic, I like the idea of not having to deal with several SDKS/IDE myself. This not considering the MacOS factor, which is actually a "huge" investment for indie developers.
In any case, this kind of build system would take a lot of work to get done, and might not worth it (for TGC).
Regards
Paulo