I got my first 10/10 win with Snipers as my bonus ships and I found them to be very useful. They were the first MarkII ships I unlocked. Both Snipers and Sentinels are very good. Good damage and survivability because of the infinite range. Also because of the infinite range their dps doesn't drop during a fight because they don't get destroyed (like Fighters for example). Sentinels' have better survivability because they're immune to insta-kills (Ion Cannons) and have more health. Snipers are better at kiting because they're faster.
Sure Sentinels have 3,1x higher base dps than Snipers but Sentinels don't have damage bonuses. They're good at destroying everything but not very good at destroying anything.
Snipers' damage bonuses are what makes them very good: 6 Light, 6 Medium, 6 Close-Combat, 0.1 Command-Grade, 6 Polycrystal. So Snipers will destroy Fighters and Bombers and thus protect your Bombers and Missile Frigates. When fighters are destroyed Bombers can destroy Missile Frigates and when Bombers are destroyed you don't have to worry about (at least not so much) losing force fields and Missile Frigates can do their job.
Um, Kahuna, I don't think a factor of two difference against 4 hull types, even if they are very common, is quite enough compensation for half vs everything else. I don't think snipers are bad, per say, just almost completely worse than sentinals. I just noticed by the way, that snipers will completely murder sentinal frigates in a fight. Snipers drop a sentinal in 3 shots = 18 seconds from engagement. Sentinals need 5 shots = 24 seconds from engagement. Focus fire makes this a lot worse between caps, especially at extreme ranges, because the later salvos will be fired with more sentinals dead, and at long enough range, a sniper may get a shot off even after it has a lethal number of sentinal shots in.
Actually, now that I think about it, the shot speed is a decent benefit, especially if you have focus fire snipers on. The problem is that it is more beneficial on the defense, where you can place snipers at the edges of the well, forcing the enemy to split forces while under instant fire, while Sentinals get all the defensive fringe perks.
Just to clarify my position, I don't think snipers are bad, just that sentinals are better at their job almost all the time, plus have additional capabilites. Snipers are certainly a very good unlock. It's just that I rate sentinals not much below wtfhax ships like SSBs, blade spawners and shield bearers.