Most I've ever got out of a sub-surface fissure was 6t of something or other.
On that basis, it's probably a bit unfair to say the SSD launcher is "pointless" but, let's face it, you're probably not going to miss it if you remove it.
If you've got an agile laser-mining ship with a free C1 slot, it might be worth fitting one.
After all, if you're getting 5t of ore from an average 'roid, you might be able to double that by mining an SS fissure.
OTOH, if you struggle with the mini-game, or your ship makes it difficult to target SS fissures, it's probably better to ignore SS fissures and just go kill another 'roid instead.
I have the SSD launcher fitted to my mining AspXs and it does contribute to what I mine, but that's 'cos the Asp is agile so it is, basically, free credits for little effort.
I don't bother fitting them to my mining Python cos when I'm flying that I'm only interested in finding VOs.
I don't have one fitted to my mining T10 'cos it's too slow to target SS fissures easily and the C1 hardpoints are better used by fitting another mining laser.
So, limited usefulness rather than pointless.