That's certainly better than nothing. At least you could pump out some fighters or something.
The problem, previously, is that colonies were untouchable, presumably until endgame. The problem now is that they are indefensible. If someone is bent on taking your new colony (whether as counter attack, or whatever), you have to be able to defend it for almost a week before it actually becomes useful to you. I have a feeling that the resources spent defending it are not going to be justified by its greatly decreased value.
One thing that bothers me about this in general is that "conventional" attacks on colonies are not part of any existing win conditions. In the current model, there is arguably no point in attacking colonies at all. "Fully grown" ones are effectively impossible to crack, and battling over fledgling colonies doesn't really get you anything (it gets you a tiny colony that will take a week to be of any value if it isn't taken back immediately).
I feel like slowing their growth does not address the fundamental issue, because it only makes colonies vulnerable at a time when they are not providing any value anyways. If killing colonies is not directly part of any win condition, the only reason to do it is to cripple a player's production capabilities. That aspect of colony combat is not addressed here, because by the time they have production capabilities significant enough to be worth targeting, they have already reached "invincibility."
Does that make any sense?