First of all, the vast majority of aircraft altimeters use air pressure to measure the altitude, so I wouldn't necessarily say they are a joke. They MUST be reset to the current atmospheric pressure (altimeter setting) to be useful. It isn't unusual for the pressure to change substantially from one day to the next. Those are the highs and lows you see on the weather map, high and low pressure areas which move across the country.

In the U.S. and Canada, we use "inches of Mercury" (" Hg) as the unit to measure barometric pressure in the aviation world. (In Canada we now use kilopascals for non aviation use, which are related to millibars, as a metric measurement of atmospheric pressure). A change of one inch of pressure equals 1,000 ft. I think in the above post where you mention millibars(Mb) you may be confusing it with inches of mercury as the equivalent millibar setting for 30.00 " Hg, would be 1015.9 millibars.

A change from 30.00 "Hg to 28.00 "Hg would be 2000 ft., which would be very unusual. It's common, however for the change to be enough to cause an error of for example 500' over a day if the weather is changing rapidly. The pressure overnight can be substantial if you've had a storm pass through and could be 1,000 feet in error if you don't recalibrate the altimeter, which pilots do periodically as they fly across the country, at low altitudes. At airline type altitudes (above 18,000') all altimeters are set to a common setting, so they will all be the same, and then they are reset to the local setting as the aircraft descends through that altitude nearing their destination, with the local altimeter setting.

As you can see this altimeter business can be fairly complicated if you want a really accurate reading. Even GPS altitudes can be in error depending on how the GPS is set and the satellites it is picking up.