I never really feel like fleet ships have quite enough armor for a 3x boost to matter anyway. Man, 3x 200? 600 armor? That'll put an awful large dent in the 20,000 damage my fleet ships do already!
More than you might think actually. Starting at around 650000 HP, armor going from 200 to 600 will let the ship last one additional 20000 damage shot. This could actually make a pretty significant difference in about same sized army fights, even if its contribution is hard to see directly. Heck, even going from 0 to 200 will let you last one additional shot from much less HP than that.
Of course, there is that in this game, you rarely have roughly same sized fleet fights between you and the AI, and most ships are so cheap and player economy can get so strong you don't really notice those extra saved ships, or lack thereof if you didn't get armor. Pretty much only starship and above really get expensive enough for you to start caring about individual cost.
So really, the armor booster, in the current balance, really only is noticeable for ships/structure with already large armor values, even if your actual military effectiveness is actually a decent contribution, due to how cheap fleet ships are. (There is that player economy thing again that tends to permeate balance discussions)
