I argue firmly against random hit point rolls.
A fighter's flat 10 HP, a ranger's flat 10, and a barbarian's flat 12 HP are part of their compensation for not being able to cast Greater Magic Weapon and so on.
The classes who have a smaller range of hit points to roll into are hurt far less than the three classes I listed above, as they rely on a high HP total to counterbalance vulnerability to magic spells, and a lower AC score than monks and rogues.
If we were to change to a system of randomized HP, the class that would be hurt the least and benefit from it most is the cleric, which is already very powerful.
The problem with NPC's having low HP can easily be remedied in a number of ways with the tools we have currently. The DM Buffing Tool allows you to add as much HP as you like to an NPC, and you can also adjust the constitution to whatever you wish the HP total to be.
Furthermore, for randomly spawned NPC's, I feel like the HP value should remain lower than that of a typical adventurer because NPC's are often more numerous, and for a horde of skeletons, we don't want each individual skeleton to feel like a PVP encounter.