Absolutely not! It's not 'them against us', it's all of us equal together. I'm not infallible and I don't want my children to think I am and would rather we all sought common preferences together. If I'm asking the children not to do something, and DH can find a reason why I'm being unreasonable, I would expect him to say so and vice versa.
Although it is absolutely infuriating when it happens, especially when you have to admit they're right.
Example: I say "Please don't play with your food!", DH says "Why not? Look at the fab volcano they're making! They're eating it too." Me: "Oh FFS, don't be so reasonable! Ok, girls, but please remember that it's not really ok to play with your food when we're at other people's houses but while we're at home then go for it".
Children learn about how to accept you were wrong - 'tis a good thing!