I understand your position completely. I was just offering my advice gained over 14 years in this biz. Just remember, the client may be happy initially, but when those things start to fail (and they will) he will probably include you when he tells others; "Oh that low voltage lighting stuff is all crap! I spent a ton of money on mine and it all failed. I wish I never hired that guy either cause he says the only way to fix it is re-install more of the same." It is amazing how quickly dis-satisfied clients will forget that you told them so, and associate you with their lack of satisfaction.
There are times, and jobs, when it is better to simply say that we advise against this type of installation and we do not do this type of work and move along. It has nothing to do with being a prima donna and everything to do with working with integrity and being professional.
best of luck.