Nothing wrong with creating an epub3 book for a client, just as long as they know that the spec is not universally supported and they would be marketing their book to clients using a specific device.
Same goes for books that work with scripting solutions supported by one vendor only.
Is this situation perfect? Surely not, but Toxaris' perpetual frowny conservatism also helps no-one.
To me, there is nothing inherently wrong with the epub3 standard, such as it is. In fact, the possibilities are tantalisingly close. Now we just have to brave the cross-currents of the standards battle and all of us ebook makers will be just fine. Know what's possible, and to what extent; then advise your client to the best of your knowledge.