I'd like to know if the rack located on top of most expresso machines is only used to store expresso cups or do they actually warm them up also? Is it best to have this option or not when looking for an expresso machine (i.e., Nespresso)?
Yeah most machines have that feature, but IMO they are useless unless the machine stays on for quite awhile and that isn't to be done with pretty much any true home machine. Your best bet would to be heat the cup in a microwave with water in it or run some water from the group or steam wand into the cup to heat it. Later!
In Italy and most of Europe
the espresso and cappuccino cups are made of ceramic,
and bars go through so many so fast (hopefully for their business) that most of the cafes uses dish washers made specifically for cleaning cups.
On a commercial machine sold in Europe, there is an optional warmer switch heated either electrically or, less-usual, with steam. Basically, this is to bring the cups back to optimal temperature before being used again. However, the heat rising from a machine's boiler will still keep your ceramic cups hot, it will just take a little longer.
PS a little peculiarity
we common mortals always have the tendency to set cups on machine upside down
which to me makes sense, keep dust out etc etc
but a serious barista in Italy will put the cup facing up
the reason being:
the part of the cup that touches your lips shouldn't be the hottest one,
makes a really good point ......