If the PPE/ISP they take from others is not permanent though... it doesn't seem like much of a drawback to these creatures unless they are somehow isolated.
Even a 15th level creature will only need 30 points per month...
These creatures do not have a listed ISP for using their psychic abilities, so I don't know if they can fuel psychic powers on their own. Did pools maybe get added somewhere in errata? I'm not sure how they would store ISP they steal. Should the 10/level they grab in a melee round be lost if not spent immediately on a power?
But anyway, they do have PPE, and even a lesser entity will have at least 1 PPE.
If the PPE vampirism is impermanent, then reducing them from 1/1 to 0/1 would be presumably be reversed in an hour's rest. Pg 126 "commonly recovered" might not apply to all situations though, so perhaps entities are an exception and they don't recover PPE on their own?
If they do, then I don't see what's stopping you from getting 24 PPE from a lesser entity by hanging out with it for a day and sucking it's 1 PPE up that it gets back each hour.
If it had 2-5 PPE, that would go even faster!
This might explain why astral entities might live indefinitely as long as they occasionally "socialize" with each other, but die off if they somehow get isolated and can't find any other entities. Perhaps why they would travel together in packs of at least 2. Preferably more though, since that gives you "backups" in case the other gets lost/dies.
Lesser entities would have an IQ ranging from 1 to 10, so some might be too stupid to cooperate like this, or get lost easily, if we incorporated Rifts Ultimate Edition's penalties for low IQ.
Even though it would make sense to be "willing" (forgoe the save vs psionics) and cooperate for mutual PPE exchange, dumb entities might also be unable to understand negotiating cooperatively like this. It mentions also, even for smart ones:
- a few are actually benign, never feeding on unwilling victims and preferring to trade services for energy
A pair of benign entities ("a few" sounds like this isn't a particularly common trait though...) could probably cooperate and exchange PPE to feed each other, but that kind of exchange wouldn't be possible with non-benign entities, either due to being paranoid or too low-IQ to understand "trade".
They could still feed each other, of course, but due to lack of cooperation it would take long to do, due to saving vs psi preventing the feeding from succeeding (entities are +3, in addition to all being psychics giving a higher base save)
Plus they might start fighting each other if they are offended by other entities doing this, so this might attract entities to the physical realm toward targets that can't hit back allowing plenty of time to suckle.
There is a question though: would losing your PPE temporarily actually be threatening to an entity? How does this inconvenience them? The only way I can think is perhaps if they were able to draw upon their own PPE and convert it to ISP to fuel their psychic powers. Given the 4:1 ratio, this doesn't seem particularly unbalancing to allow, and maybe this could serve in lieu of actually having an ISP pool?
If that capped ISP costs it would be pretty limiting though, as lessers could only fuel 1 ISP powers at best with 4-6 PPE (none at all with 1-3), commons could only fuel up to 4 ISP powers with 12, and greaters up to 7 ISP with 28-30. Given that Greater Astral Entities have access to "master" level powers, a lot of them wouldn't be fuelable at all if relying just on converting their own PPE to temporary ISP, meaning they could only be used with a fuel source around?
That makes me wonder: is the 10/level cap for a sole target, or could you have a level 2 GAE with a posse of 20 LAEs (each with 1 PPE) draw on all 20 of their PPE in a single round and convert it to 5 ISP?
If he was limited to 1 at a time, then 1 PPE wouldn't amount to any ISP and be of no help to him if it faded away by the next round he could feed on the 2nd posse member. Unless perhaps stolen PPE could be retained for longer periods.
If we borrowed on some of the rules that mages use in other games (storing PPE above their maximums) for entities then this might make them a little more flexible here.