Not telling them of guests purposefully is dishonest IMO. I have had several situations where we needed extra keys and a couple of situations were we had someone stay overnight that we were not expecting. In every case, we let the front desk know and got keys. In every case we were on the
DDP and not once did the subject of paying for the plan for an extra person come up, except when I asked about it myself and was told they couldn't if they wanted to. I am not aware of a single instance where one was told they would have to pay for an extra once already checked in or at the time of check in. Like so many things it comes down to intent. If the intent was to get a person in without paying the DDP, it's wrong to do so, if not and life happens, I'd vote the other way. Timeshares are different than hotels and it's far more likely to have people come and go during a stay. At some point
DVC/Disney will have to deal with the catch 22 of having to list the DDP early and pay for everyone for the entire time vs not being able to add/change at check in. Until then, the rules are what they are and honest people will try to follow them.