Blobs are for intercepting bombs. They don't do **** to capships. Even Flak does a better job.
Well in FS1 they were more for caps. Those blobs were a threat to the Omega transports fleeing past the Anvil and the blob turrets were in the front of the Typhon, and capships used them to duke it out in FS1.
Well, yes, Blobs were anti-capital weapons in FS1. But that's all capital ships had, except for the odd missile launcher.
And yeah, I guess the weak, unshielded non-combat craft could be killed by blob fire, but Flak is generally faster at it.
Blobs are excellent anti-bomb weapons though, as they don't miss like Flak does (flak explosions do not affect bombs), and fire much more often than AAA beams. In FS1, where ships had a lot more blob turrets, taking out weapons is very useful, if not necessary, when doing bombing runs. FS2 mixes it up a bit by adding more things that threaten the bomber, replacing the things that intercept the bombs.