As you can see in this page: http://api.haiku-os.org/classBString.html you can’t add a long long, a double or a bool to a string, if you want to do this you should cast to a lower type risking to lose precision.
Seeing the BString code it seems easy to add this methods as in the end snprintf() is used.
They are not added because it could broke BeOs compatibility?
What do you think?