A few reasons I can think of why I wouldn't do it:
1) extra work for no forseen reason or benefit;
2) the bone acts as a doneness indicator, allowing you to see the true shrinkage of the meat, as well as determine if the connective tissues have melted away by being able to pull the bone from the shoulder as a test;
3) the bone adds flavor to the meat as it cooks...real meat stocks are flavored with bones as the base
4) cutting the shoulder open presents me with three questions:
4.1) am I going to reclose the roast before cooking, and if so, can I get the meat's internal temp up high enough soon enough to be safe (being it is no longer a whole muscle meat being cut and closed);
If I leave the roast butterflied when I cook it:
4.2 am I willing to accept that the cooking time will be greatly reduced which can in turn lead to a less tender finished product?
4.3) am I willing to loose tons of natural juices during the cooking, being I increased the actual surface area of the pork shoulder?
Too darn complicated, with too many reasons why I wouldn't de-bone a shoulder, so I won't.