Unless it's something you want to add to your portfolio, give them CS.
It may look dreadful to you, but the client couldn't give a fig. I always match the spec, no matter how ugly it appears. I've put together some absolute monstrosities in my time, but happy clients mean repeat business. I find trying to impose my professional opinion on those that have hired me as a paid monkey just costs time and ultimately money.
Unfortunately true. A few years back when framesets were all the rage, I tried to talk a potential client out of using them during a pitch. Lost the pitch, a very lucrative contract, repeat business and any recommendations I might have got.
If you can afford to refuse work, that's one thing, but when you've got to put food on the table then sometimes you've got to lock your design ethics away while you bag the money.
It's always worth voicing your opinion and giving the client actually what they are paying for, put just be careful not to over step the mark. The customer is always right
(sometimes).