Unfortunately, many saw manufacturers do not provide a long enough arbor to accommodate a full 3/4" dado set, and I believe this is for a reason. A table saw (or radial arm saw) must have a strong motor to handle a deep cut with a full dado set or the motor will bog down under the load.
If the motor's RPMs drop too far, the chance of kickback increases. Because the saw's motor isn't designed to handle this type of load, I believe the engineers intentionally employ a shorter arbor to prevent users from employing the full dado set with both the arbor flange and arbor washer properly installed. The problems is, the user might be inclined to try and use the dado set without the arbor flange (on the motor side of the blade) or the arbor washer (on the nut side of the blade) to try and employ more chippers in their set. This is a very dangerous decision, as the arbor washer and flange are in place to help stabilize the blade. You should never use your saw without both the arbor washer and arbor flange properly installed. Consult the owner's manual for your saw to learn more. You should also never install so many chippers that the arbor nut is not fully threaded when tightened. In other words, make sure that the arbor fully extends through the arbor nut. If you cannot see that the end of the arbor extends at least minimally past the arbor nut, then the nut isn't fully installed, and would present a potentially dangerous situation if you were to proceed with the setup. Remove a chipper or two and re-attach the outer dado blade, arbor washer and nut. Now, with a short-arbor table saw, you may be able to safely use only the two outer blades of the dado set, along with a chipper or two, giving you a 3/8 - 1/2" wide cut. What do you do if your woodworking plans call for a 3/4" dado? Simply align the fence and cut one side of the dado (or rabbet), then adjust the fence and make a second cut to complete the dado. While it takes a bit longer to make the dado, it is less taxing on your saw's motor and much, much safer than using an improperly installed dado blade.
