Do you mean ultra flex thinset? Expansion joints are required in certain installs and those are defined in the TCNA and ANSI specs. The owner may not want expansion joints, but in order to have a lasting install they are required. If the GC and owner won't agree with you on proper installation, then it would be best to pass on this one. Otherwise, the conversation about how they didn't want the joints will be long forgotten and you'll be stuck with the replacement because you are the "tile" professional.