>> On most "modern" cars (15 years +) which are designed to be used with a remote key fob for unlocking, the alarm will be activated if you use the mechanical key to unlock the doors. To silence the alarm you then have to insert a registered/paired key into the ignition switch pdq.
Define "most", plesae. Most "modern american" cars? Because this does not hold true for ANY of the modern cars I have owned / driven in the past and had the need/curiosity of opening mechanically. That includes a 2006 Alfa Romeo, a 2003 VW, a 2012 BMW, a circa 2010 Audi A6. There is a trigger switch in the door lock assembly that will immediately trigger the alarm if you try to turn the lock brute force or if you try to pull the core out. But using a properly cut key to turn the lock without force has not set off the alarm on any vehicle I have ever seen (here in Europe). It in fact not only doesn't trigger the alarm (because door is opened) but it disables the "movement detection" sensor in the interior on all examples.
>> It actually makes sound logical design sense, as a mechanical brute force on the lock, or a forged key will trigger the alarm.
You are talking about two different things. Brute force (screwdriver) will sound alarm. A forged key is indistinguishable from a real one as far as the mechanical doorlock assembly is concerned. If the cut is right it fits.
The opposite is correct, if you have the legitimate key, but the keyfob lock/unlock is broken it would essentially mean the car cannot be used if it was indeed wired / setup like this. You could open the door with the mech key but the alarm would be blaring for no good reason at all.
You have to allow for complete failure of the keyfob electronics (dead battery, broken electronics) or you will have pissed customers swearing on $DEITY that they will never again buy a car from $BRAND. That is why the mechanical key is still part of the fobs. And the transponder for immobilizer is separate from the keyfob electronics and will allow starting of the vehicle so customer can haul himself home and replace battery or buy an overpriced replacement fob from dealership (and on being quoted the price he will then swear to $DEITY to never again...)