Light ships are super cheap and its frankly impossible to put much money into them. Whether you should have one or not is a matter of personal preference, they are a lot of fun but lack the power of bigger ships. I wish I could have one of every ship model in the game but alas, you can only have 3. You need to make a plan once you get medium ships as far as whether the ship is one you will keep or just a stepping stone to a war class or a trial to see if you like it before you dump big money and mats into upgrades. With the light ships you can buy and sell with little consequence so long as you aren't selling an expensive ship just to make room in your inventory.