It is not a question of client preference.  If there is community income, use Form 8958.  If it isn't, the clients should have some evidence of that if they are residents of a community-property state, and then they can report just their own income.  Since separate returns usually result in more total tax being collected, IRS is unlikely to audit them in order to issue a refund.  But there is really only one right way to do it.  

On the other hand, MFJ should not be the mindless default.  Too much time is spent by too many taxpayers and advocates, on "innocent spouse" cases, Preparing tax returns should not require training in psychology and counseling, but unfortunately the risks can't be ignored.