You walk into a restaurant that serves sandwiches and hamburgers and tell the counter staff, "I notice that your business is a little slow right now. How about making me a pizza or grilling some chicken?"
They will say no.
Many businesses in other disciplines, however, rush for the ingredients. Their view is, "If the customer requests a product or service and it is roughly in our area of expertise, we will customize and alter in order to please the customer."
Sometimes that works. On most occasions, however, it is a time-killer, a money-waster, and a diversion from the main mission. You've heard of mission-creep with regard to the military? There is also mission-creep in business. An alteration here and there and you can wake up with a bunch of new products that were designed for a small fraction of your customers.
Now if you charge a hefty fee for customization, you may come out ahead in the short run. The larger question, however, is whether the distraction from the larger picture has strengthened or weakened your efforts.
It can be difficult to say "That's not what we do" and watch business stroll out the door but it can be one of the wisest decisions you make.