I can appreciate the philosophical approach to your opinion, but there are a few problems with it.
For example, I loathe soy milk. I feel like most of us in the coffee industry do. But some customers need it for dietary reason (e.g. diary and nut allergies). Should I not serve soy because of my opinion?
Taste is subjective, as others have mentioned. So if I base my decisions of what to or not to serve for customers based on that, I am trying to make that customer a slave to my preferences. Here is a great piece on TED Talks about spaghetti sauce preferences. I highly recommend it. Malcolm Gladwell on Spaghetti Sauce
How can we, as coffee professionals, take a stand on issues? The only way to have a hard stop on what we will do without coming across as offensive to the customers is in the menu creation itself. So if I don't carry toffee nut syrup, then it is not offensive to say I won't put it in a drink. I can't put it in. But if I carry decaf, then half-caf is fair game... I don't like it, but it's a thing. And also as previously mentioned, some need it for medical reasons.
The other exception would be similar to the example that @FelixMcCarthy shared with the mixing of beans. That would cause a breakdown of tested procedure. Who knows about the solubility, extraction, etc...? Also, we have some drink combinations that just do not work. For example, one of our syrups causes the half and half to curdle for hot drinks. If someone asks for that (obscure) drink combination, I explain that it just doesn't work to make a good drink.
So yes, there are times to say no, but I can't base it solely on my perception of a "good" drink. Even Apple, who is a great example of curating an experience and limiting options, appreciates that not one-size fits all. They have several options to accommodate differing needs and desires.
Also, atypical requests should challenge us to be excellent and stretch our skills and knowledge. For example, @MattPerger has done research on how to make non-dairy milks better in the steaming and drink-crafting process, but most of us would agree that good, higher fat cow's milk is best. How can we be better in adverse scenarios? We're forced to grow and be more creative because of the challenges.
Lastly, making choices for business reasons is a thing. It's what pays your salary. I wouldn't be so quick to judge before you've tried the shoe on the other foot. I don't like caramel lattes, but they keep the lights on... and they open the door for dialogue in the future!