I think its best to not only say "i don't recommend it' but to also give a business reason for why you don't.

For instance, if the only thing it does is throw off the aesthetics off the page, you could say add "and if the aesthetics are poor, people will not see your site as that belonging to a serious business".

If you bring it back to the business-aspect people are more liable to listen to your expertise, as opposed to having it just be an artistic claim.