Ive removed the jack inserts/points (if thats the correct term?) on the longs since they were rusty. The longs themselves are good. Is there really any reason to weld new ones on since I wouldn't use them anyway and it's not going to be a concours type car.

Only to make it look finished. Most Rocker panels already have the hole cut out. I suppose you could fill that solid too. Just make sure you remove all the rust and seal.

I have been told that they reinforce a critical fkex area on the longs from flexing.

Maybe the plate part does.... I cut the actual jack points off of my old car and left the plate. It would make sense that the plate part would add some rigidity.

Interesting, maybe it stiffens up that area due to the fact there's a large hole in the long right behind the jack point.
Does the plastic stopper plug into jack point or into the rocker panel so in other words if the jack point wasn't there how could the plastic stopper be held in place ?

The plastic cover plugs into the actual jack receiver hole - snugly.

Brad is right, weld them back in!!!!!!!!!

Why not use them? If the car is solid it should not be a problem.
I use it every time. Jack it up using these points, then place the Jackstands