In my line of work, we frequently change the drawings, maintaining a set of drawing revisions including any xrefs works well for us because we deal with people, and people make mistakes and change their mind.
Take for example, one client who wanted their plans in a certain layout, after revising the plans 3 or 4 times, the final result was that the client wanted to revert back to an earlier incarnation. Keep in mind that when I speak of revisions, I am not talking about corrections, I am talking about things like .... move this wall, change that cabinet, move this wall, stretch this end, move these lights, add this widget, etc... when the client gets the drawings, they frequently, like 85% of the time at some point want to revert back to an earlier incarnation. It is for this reason alone that we maintain these files. It makes our job significantly easier. If we were not dealing with outside clients, it would basically be a WYSIWYG situation, so a revision history becomes unnecessary.
Imagine if one of your clients came back 6 months after you have completed a job and told you that he wanted to go all the way back to the second set of revisions..... If you have no revision history, you would have to aquire a set of the drawings of that revision and recreate that drawing, if the changes were significant enough, you would relish the thought of having a drawing to refer back to. Keep in mind, the client has already paid for this service and he may well expect that if you have already done the drawing previously, it is available. He will not want to hear that you have to recreate it and therefore incur additional expense.
So, I guess I have the best reason of all, it makes my job significantly easier, the boss is happier, the client is happier, and we get more work because of it.