There are a handful of instances where it's useful to build an antimatter power plant, but yes, they're generally useless and only worth building when you can use them to boost a structure that gives a particularly strong level bonus. Hyperion Shrinkers are the main offender; if the level bonus of the Elerium Defense Shield has been corrected or gets corrected so that the level bonus is actually applied (the level bonus hasn't been applied in any version that I can remember, though I haven't checked the current version yet), that'll be another big offender, as the XML defines the level bonus to be an empire-wide planetary defense bonus, and it'd arguably be an even worse offender than the Hyperion Shrinkers are since an empire-wide planetary defense bonus of 5% per level can render your entire empire immune to invasion with only a relatively small number of Elerium Defense Shields.
