Are point defenses actually helpful vs sentinels? I hear they prevent missile attacks, but that the missiles can't really kill you anyway.
Absolutely.
Course, they're only useful if they're mounted on a ship that can land somewhere within the ruins, themselves.
If you go in an Annie, and have to park a couple of km away, they're not going to help.
Small ship FTW.
The missiles
are fairly dangerous, simply 'cos they can flip your SRV over quite easily, which means you can't defend yourself when the Sentinels attack with lasers.
And then your SRV explodes.
And then you have to go all the way back to the nearest station to replace it.
On a related note, when you're fighting with the sentinels, always face them, always put your handbrake on and then swap to the turret to fight them.
Your SRV is more stable along it's front/back axis so it's harder to flip over if you take fire to the front.
If the Sentinels hit the front of your SRV they'll push it backwards and you can keep shooting back at them.
If they hit the side of your SRV it'll flip over and you'll be defenceless until you can get back on your wheels.
Equally, with the handbrake on your SRV won't move around as much when you take fire from the sentinels.
Once you know what you're doing, the Sentinels present almost no threat at all.
If you're not familiar with them, though, the main danger is that they cause confusion and then you panic and allow bad things to happen.
Try to fight them on YOUR terms.
When you see one, just drive around in your SRV (they're not terribly aggresive so they don't really chase you) until you're in a position where you're happy about shooting at it, swap to the turret and start shooting.
When the sentinel starts to return fire, swap back to the driver seat, change position and have another go.
If your shield goes down and/or you take hull damage, head back to your ship, board it and your SRV will be repaired, then deploy it again and have another go.
Ensuring your SRV isn't destroyed is THE most important thing.... especially if you only have one.