There's an old phrase that goes "the customer is allways right". I freaking hate that phrase, but it is true.
You can try an convince him on having a sign, a wrap, a bussiness card done in good taste, sometimes it will work and they'll listen to you. But if he is hellbent on the Serpentine/Rapier combo with a hammer for is logo 'cause he has a construction company and he likes the Serpentine/Rapier combo and a hammer makes it clear that he is a contractor. (Ok he wont call out fonts like that, but you know, like the sign on the garage on the corner of so and so). Then Serpentine/Rapier it will be, with a hammer.
Maybe it's not something you will put in your portfolio, or even tell your girlfriend you "designed" that when you drive by, but if the customer is happy, he will tell others that you did it, that you did a good job, and that they had a good service.
If you try to impose your good taste upon him, and make a better looking sign, wrap... he might not be as happy and trash you.
Just my opinion on the matter...