drop bear
Sr. Grandmaster
Well, I think that those who are truly skilled and capable with multiple systems and can utilize the different and possibly contradicting methodologies seamlessly are rare. In most cases, the skill level is not as high as the person tends to believe it is.
Overall I recommend against it too. However, there is some sense in training in several different methods in order to figure out which is best for you. Then focus on that and excel.
If you have to deal with more than one striking system then you need to have more than one system in your own tool belt.
Or to use your analogy. You pick up a weapon you should be able to shoot the thing.
People who are capable with multiple systems are the product of training multiple systems.