I have a feeling its a function of the difficulty level, the higher level you go the more active they seem to be, and the more defended their planet. They are still weak thats for sure, but I only see them stacking non combat ships in orbit at (say) Painful Level and below - above that they seem to be more active, even seen them colonising other planets on many occasions.
I also have a sneaking suspiscion they are there as a source of Tech (always strip them of techs by buying just before you take them out - great source of techs and cash from selling yours with no danger, as you are taking them out the same move), and if you take a long while getting at them, at the lower levels they then to seem to start stacking non combat ships. Almost a default mode waiting to get zapped, a game mechanic basicly.
On current game the Drengin AI declared war on one in the middle of a major war with Archeans - which was daft as the Minor was out of Drengin range, and Dregin were having a major bust up elsewhere. I had actually "forgotten" to take out that Minor, and the Drengin declaration prompted me to have a look, and realised I hadnt zapped them - which I promptly did. Its almost as if the game was reminding me to go take them out - it was certainly pointless Drengin AI declaring on them as they were out of range.
Strange .......
"we now return control of your TV set .... do doo do dooo do dooo........"
Regards
Zy