They may have reasons for making a decision that you are not privy to. Instead of giving in and telling them how great their idea is when you don't understand why something is the way it is, ask. The worst they will do is not answer you. Again, you haven't acquiesced without politely making your voice heard. Think before you open your mouth.
Motivation is key.
If you have something to say that will make a significant contribution to the conversation, go for it. If not, maybe it is best left unsaid. Realize that most people don't really have a stellar opinion of someone who strokes them. Speak your truth in a kind, thoughtful, informative manner and you will earn a lot more respect than you will for leaving a perfect lip print on your client's buttocks. Let it go.