You can build situational ships like point defence ships as needed then either refit them or mothball them after the threat is gone so its really not more expensive. I usually make point defence ships out of my latest and greatest ship size because after you kill all those fighters or destroy a lot of missles you get a big boost in your ships experience. Then just refit that ship to use your best beam weapon and it will rarely miss. So bring on your fighters and missle ships. It just helps me control the galaxy
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