@ rafi - in v1.1, the carrier frigates will be able to hold 2-3 squadrons, the same as the hangers. And so what if hangers have more armor or don't take up fleet supply. The carriers are mobile, and I would rather have a mobile force anyday.
You're somewhat comparing apples and oranges here. Unless it's earlier in a game and resources are in short supply, you will still need to build defenses on your important planets. If it's early in the game and you have some extra capacity, then sure, you can use ships alone for your defenses.
However, once you need to grow your fleet and have it be more mobile, upgrading your fleet capacity can become rather expensive. As such, it is far cheaper to build some static air defense so that you can use more of your fleet expanding while equally keeping your fleet supply as small as possible for as long as possible, thus increasing your net income.
Make sense?