You get a power grid bonus when combining a guardian PP and distributor.
I disagree they are "basically useless" though. Abridged version of a post elsewhere, on a one-to-one comparison sure, a single Guardian PP is always going to be inferior to a single G5 engineered PP. But the Guardian items are quite superior to vanilla, unengineered/low-mid engineered counterparts, and when you look at outfitting say, 10 different ships with powerplants, Guardian equipment is way more convenient to obtain than G5 engineering, due to the one-off material requirements for permanent access to guardian items, as opposed to the material requirements needed every time for G5 powerplant upgrades. So... sure, Guardian stuff might not be superior to a G5 engineered PP, but it's a perfectly adequate substitute (corollary: I never have heat management issues with them).
On an aside, I'm surprised given that stance that you think the guardian HRPs are useful. Understanding it's the only way to get Corrosive resistance... the 5% corrosive resistance can be outperformed by a G5 heavy duty modified HRP, with a greater-than 5% overall HP increase.