A 210 has posts underhood for connecting jumper cables. It's quite easy to use those to power up the car while changing the battery (it's under the rear seat). Just connect cables to another battery - car, motorcycle, etc. No need to have the other vehicle running. This will save having the reset the windows, radio, etc. - though I don't think it's any less work.

The 2nd battery was introduced with the w211 chassis. It doesn't buy you anything when changing the main battery - it's electrically isolated when the car is switched off.