This is one area I was not pleased with in RS2 at all. These units are expensive and clunky, and I had an idea to make them better\more useful during development, but it got shot down. Now, since I seem to be the only one left, I guess I get to do whatever I please.
Anyway, there are a number of problems with all of the equipment units. The worst one is how they work on the battlefield in terms of the way the units move. If you have more than one piece of equipment in a unit.....say, a unit with two or three onagers\Lithobolos....the unit has to move as if it is tied together with ropes. This results in the unit getting stuck on battlefield objects, or, even dumber, on itself as it tries to go where you tell it to go. Sometimes the unit gets so stuck it will not be able to move at all, and if it tries to fire, will actually shoot itself because one of the pieces of equipment is in the way.
I suggested to solve this by limiting the unit to one piece of equipment, and tested it a bit to see how it would work. But the team didn't like the idea, and I never had a lot of time to REALLY test it out. However, I have been testing this, and I have to say that it works marvelously. I made each unit have one piece of equipment (except the large Litholbolos, which I have to figure out why the game won't let me give the unit only one), and gave the unit 40 men. This results in much 'cleaner' movement on the battlefield, no getting stuck with each other, and a much easier time navigating those areas on the battlefield where the equipment is not allowed to go. Also, as the equipment is configured with a 'mass', the units seem to be able to move it\them much easier and quicker.
I also gave each unit larger damage figures and more ammo to make up for the loss of the other pieces of equipment, so when attacking a city wall, using one Lithobolos doesn't take any longer to bring down a wall than having two or more in the unit. Having 40 men in the unit as well, as opposed to 8-12, makes this a more viable unit once you are done using them. So it's not just a throw-away unit, and with better stats, the men actually hold their own and fight well enough to pin cavalry or other units. So they are much more useful and 'worth the money'.
The whole issue is one that CA has apparently not dealt with yet, as I don't feel this equipment getting stuck on things is 'fun'.
As far as the use and\or the usefulness of scorpions, I rather think these weapons were, even in reality, no more than a 'shock and awe' tactic. The projectile was vicious...could kill even multiple men....and just seeing it skewer someone next to you was probably a pretty frightening sight. So the weapon itself has good uses, but as the OP says (and others) the AI is not very bright when it comes to being pummeled by distant artillery and stuff. I have even tried to solve that issue by changing commands in the formations so that the AI armies are ALWAYS set in an 'attack' mode....but it seems to just get turned off as soon as the battle starts.