Doing IT in my “other life” and Sales would come exactly from the same perspective: “why don’t you just/simply fix it for the client and discuss your nerdish things later?!”, and the trouble is that neither Sales, nor client can define the problem, they simply want “the fix” and ideally for free 
I don’t feel withholding information is honest
Of course, I tell them the information in a tactful manner
They deserve to know, in my opinion
But nobody’s opinion is exactly the same, as has been pointed out many times on this very forum
No matter what you do, somebody’s always going to say you were wrong, you should have done it differently, etc.
One of the problems with replacing stuff to diagnose a problem, you may inadvertently remove the original good part, and install a faulty part, and end up in a worse position than when you started. Sometimes the replacement method makes sense though, especially for a diy’er when b/c of the nature of the problem there’s little chance of making the problem worse, and the replacement part is inexpensive and easy to replace.