Jim, as far as I can tell, it's not. But it has always seemed to me that failing to comply with the RFM in any way contravenes the principle of good airmanship. Apart from the obvious, insurance companies require it to be complied with and in the event of any insurance claim, I have no doubts about them trying to find good reason not to pay out.
However, discussion over this tragic accident has shown that some might believe that flying to performance Class A/1 (or whatever the fashionable term might be this year) is mandatory for all flights. For private operations it isn't. An alternative departure profile may be safer in conditions of very low visibility, especially from a confined area. In other words, blind adherence to alleviate one perceived risk (engine failure) may increase risk in another way (bumping into something).