I think they are absolutely worth it particularly in the early game. I was recently 15 years into a slow research game (15% speed) and I had an NPR fleet just show up and started blasting civilian ships. I was clearly out-teched and outgunned by them (15 warships in the 5K-25K range carrying gauss cannons and railguns and several large troopships, 7,500KPS speed). Against them I had a small fleet of Frigates (5K, 10cm lasers, 3,000KPS), 36 obsolete gauss fighters, 24 FACs with Box Launchers, 4 large orbital monitors around Earth (20K, no engines but huge shields, armor, and lots of small railguns), and 30 STO units (x2 12cm beams each). My forces met the enemy pretty just short of in orbit.
My frigates died without scoring a single hit - I don't even think they got in range. I timed it wrong and my fighters got there just after the Frigates were blasted and I lost half of the fighters before they even got in range and the rest weren't doing much damage at all. My FACS launched their salvo of size 6 missiles (144 missiles) and they managed to destroy two of the smaller warships and damage another but now missiles expended, they were no longer a factor. The fight moved into orbit and the NPR started to go to work on the Monitors. The Monitors were scoring hits the whole time with their rails but it wasn't enough - but they were soaking up lots of damage and it took each one a minute or so to die.
The champs though were the STO. Once the enemy fleet got in orbit, every 15 seconds 60 lasers would open up and completely destroy or cripple an enemy warship. The Monitors were obviously going to die (I lost 3 and the last one was essentially wrecked) but those STO lasers were lethal. The whole orbital battle last less than 5 minutes but the NPR lost all but 2 of their warship and my surviving fighters managed to run down a couple of the troopships which were now heading away - their only contribution.
Bottom line, I would have lost against a spoiler but those STO weapons were absolutely a lifesaver. I think they allow you to survive until your technology catches up and you can build a credible fleet.