Here's one from a vendor I currently integrate with - have multiple APIs (in this case, a SOAP one, a REST one returning JSON, and then some language SDKs that wrap the latter for convenience), and have their feature sets be an interesting Venn diagram with various intersections but no unions. Just today I found out that the in the extensive integration we've written using the SDK (which wraps the REST API, remember), I will have to shell out and use SOAP because one (1) parameter on one (1) object in their extensive object hierarchy isn't supported by REST or the SDK, just by the SOAP protocol. Nice.