Simple, apart from the 1 heavy cruiser, the others are just support units, or diplomatic envoys. Theres also the problem of having to research them, and even when u do u face the problem of temporarily depleting ur fleet to upgrade it, which is very bad when ur attacking or under attack. Although it is true that they do give u more bang for your resources, it might not be needed or be useful to you enough to scrap ur current Disciple fleet to get, say a Crusader.

I'm late to this party, but I was going to point out that we all live on earth, and because of gravitational effects, at least 80% of those digits of pi are wrong. 

However, I also like to fact check such a post, and the internet disagrees. Pi is still pi, but if you attempt to measure it,  you will get some other number. Not that you could measure anywhere near that many digits anyhow. 50 digits are enough for a circle the size of the observable universe with an accuracy of the planck length (theoretical minimum size).

As far as the actual topic, most cruisers barely shoot, they're there for the special abilities. Heavies are the exception, and others have made good cases against them...

I never have the time to build cruisers before I am overrun by millions of junky enemy frigates.  So I tend to build millions of junky frigates myself.

Aliens are the shit