If the ship benefits proportionately less from installing an excess number of these than it does from installing the right amount, that is an example of diminishing returns... whether it's because the systems that draw power become overloaded and excess power is no longer beneficial, or the heat sinks become overloaded and drawing excess power starts to do more harm than good.
This is basic stuff, yes... but, a few posts ago Zack said there was nothing stopping people from using more. Apparently, there are several things that stop people from using more. So, uh, that concern is resolved, in a kind of roundabout way, but something could still be put in the article to reflect it, like this: 'As with any other power source, installing extra taps on a ship can provide a backup power supply in case of emergency.' It can stop short of spelling out that this is the only benefit (unless you want to risk frying your systems) since the line implies that this is the only practical benefit.
I definitely think a modular system like this should have some serious inefficiencies compared to power plants that are designed for, and built into ships and other devices expressly to power their systems (or in the case of cheaper stuff, where the ship or device is designed around its power plant), although since they're essentially magical, they could still be more efficient than antimatter or fusion reactors. I can't imagine how economies of scale work for aether reactors.
One area where these might be weak is powering systems that take extreme amounts of energy to use, since it may be difficult to route power from every tap through one place instead of distributing it over the ship, given that they're a multi-point power source.