Sure, smaller ships may not be useful for attacking colonies, but you have more than a "tiny window of opportunity" when it comes to outposts. Outposts don't generate population, so their defense doesn't change over time. The only time it changes is if you set up orbital defenses (minefields etc). Otherwise, outposts are easily captured with a small fleet of small ships. If someone has 5 colonies and 20 outposts, and you go capture their 20 outposts with small ships, they're not gonna be getting much income to do a lot about it while you work on building your bigger ships to kill their colonies.
I would agree with this if cruisers could not be auto-converted to beam defenses. Since no one really knew about this hidden feature, there was a fairly large window in which a lot of back and forth with little ships happened.

Now I know I can rush cruisers, capture a bunch of outposts and insta-protect them all in less than two days. Mammetum is going to be a whole different ball game.