The most annoyingly untrue tenet of business is the adage that "the customer is always right." 

They're not.  And embracing that adage can mean failing your customer as well as yourself.  What is true is that the customer always has needs.

The best vendors understand that and are willing to say no when it's warranted.  Usually in more than one syllable and in a way that benefits your business, but ultimately they excel by helping to understand the root need, and find the best way to meet it within agreed constraints. 

Customers do business with vendors because there is something they cannot or will not do, whether it's hiring a dogwalker or a mobile app developer. And it's not just with external vendors; it's also the skilled workers within an organization.  If you're unable to leverage your expertise -- as a vendor or an employee, then you aren't really helping anyone, and are probably rather frustrated.  As a customer, if you aren't allowing experts to actually use their expertise, then you're just wasting money on minions. 

No one really has unlimited resources these days, at least outside of dreamland.  And do you really want someone who just does what you say, or actually cares enough to understand what you need?

That's not to say that an expert shouldn't be questioned. If you've hired an actual expert they should be able to explain strategies or decisions. And you shouldn't expect them to do otherwise anymore than you should hire those dreamland minions.

There are more advantages to a vendor who doesn't acquiesce to every request; they can also save you money, time, and even risk exposure.  The best solution is often not the most expensive or even have the most bells and whistles.  A good expert will take the time to understand your requirements and find the solution that best suits your needs.