I am wondering if it's entirely necessary to mortise a door and jamb for hinges and other miscellaneous door hardware. The only thing I can think of is that, in addition to being screwed in, the mortising increases integrity by locking the hinge in place provided that the fit is tight. Outside of that, it looks like it comes down to aesthetics and just looks nicer.
Well, let's take hinges for example. They typically have plates 3 or more millimeters thick and with two such plates it means you need a 6 millimeters gap between the door and the frame to accomodate the hinges. Do you really want a 6 millimeters gap? It will conduct noise, let unwanted cold/hot air through the door, it'll be very easy to put a crowbar and break the door open.
If you don't care of having a huge gap - no problem, you can install hinges without making mortises for them.