There IS a tactial reason for doing it. You know the actual reason why they are called 'hot' drops in the first place.
If you are dropping into a 'hot' zone, ie a zone that is still contested or occupied by enemy forces it makes sense for an airship to drop its mechs before touching down for two reasons.
First of all if the enemy still has surface-to-air defensive capabilites, one dropship and 4 mechs in the air make 5 targets alltogether compared to just the dropship (1 target) dramatically increasing the chances of any mechs making it safely to the ground. Obviously if even more dropships and mechs are present in the air this chance goes up even further. Think about airborne infantry regiments parachute dropping en-masse behind enemy lines for comparisons.
Likewise airdropping mechs makes sense if you want to get the drop ships out of a hot zone really fast. It takes much less time and fuel to drop mechs from the air, hence increasing the chances of any dropships leaving the area intact after completing said drop.