Actually, for every planet class from 14 through 19, +5% PQ gains you nothing at first, but gains you 1 point of PQ over no bonus as soon as you add Soil Enrichment and/or Habitat Improvement, so it isn't useless at all. Starting at PQ 20, you get one level right off the bat.
I've got a page with a chart on it demonstrating this at http://geekzilla.org/galciv/pq.html if anyone wants to see it for themselves.
So, did they change the price of the 5% PQ bonus, or is it still 3 points?