Against good competition (read as "real human opponents and not feckless AI), a capital ships-only fleet would get squashed.
Suppose that you could have 800 fleet supply of any ships you wanted. Would you rather have, say, 133 Vasari Assailant LRMs or 16 Level 10 Evacuators to nanite them? The Assailants would demolsh the capital ship fleet.
Even a fleet of 16 Level 10 Marzas that could missile barrage would probably fall to a fleet of 800 supply's worth of carriers with bombers. The carriers would just keep away from the Marzas.
Basically, capital ships have utility value for your fleet. They can colonize, bomb planets, and sometimes do other things (shield restore, nanite enemy ships, heal, repel strikecraft). It can be very dangerous to them to bring them into large battles simply because they are large and expensive targets and large enemy fleets would like nothing more than to focus fire on them. Ironically, it's somewhat harder to use them later in the game than eariler for that reason; they're more useful against small fleets where they aren't in as much danger.
Anyway, come play the game in online multiplayer and you can learn more about these issues first hand. You'll also get to enjoy more challenging competition, having human allies, and perhaps making friends and the camraderie of other poeple.
