[Q]

Fighters are extremely valuable IMO.
First build them whenever you have spare resources. You can store the fighters and you don't have to pay maintenance. They are a very good defence for colonies (same reasons as above) and in battles between large fleets, that would be otherwise more or less equal, they will decide who wins. They give you a important additional fire power. And Last but not least the famous allegiance subverter does not work against fighters, so they are a must against psychic races.
However you will always lose more or less of them in battle. Therefore if I have enough ships to defeat the enemy I don't use them but keep them as reserve.