Correct, the MR74 has 2x5GHz band specific ports and 2x2.4GHz band specific ports. The ANT-20 is a dual-band omni antenna, so they can and should be used on all 4 ports of the MR74. Perhaps one exception is if you have the 2.4GHz radio turned off in the radio settings (you actually can turn off the 2.4 GHz radio by setting the Tx power to "off" - don't use 0 dBm because that is still 1mW) and then you may not have any antennas physically connected to the 2.4GHz ports since the 2.4GHz radio would be off. But generally speaking, you should always have a physical antenna connected, especially if you're running dual-band SSIDs.
Theoretically, yes, there is possible harm to running "naked" antenna ports. Whenever there's an impedance mismatch, there is a signal reflection, causing an energy loss, and in the world of RF, that energy loss is actually in the form of heat. It deals with "match" meaning impedance match and gives rise to the term "thermal impedance", basically a measure of heat loss in degrees celsius per watt, so it's sort of an indicator of how much the equipment will heat up.
With no antenna, you've got an "infinite" resistance where it's expecting a 50-ohm terminator (the antenna). So instead of a typical VSWR (Voltage Standing Wave Ratio) of maybe 1.2:1 or 1.3:1 (close to the theoretical perfect match of 1:1) you now have an infinite VSWR with no antenna, and a full strength signal reflection heading back towards the transmitter. Such a high VSWR can damage RF equipment by (eventually) burning out the transmitter’s electronics, like output transistors. This is typically not a catastrophic failure all at once, but a cumulative one that can cause the equipment to fail prematurely.
Note that when you order ANT-20 there are 2 physical dual-band antennas in the box, they come in pairs. So for any dual band AP like MR72/74/84 you would order 2 ANT-20 which would get you 2 pair, 4 physical antennas for the 4 physical ports.
Hope that helps!
