Astute observation re. the lack of movement of the spring guide if the stock tube is screwed into it...you are correct...sort of, sometimes.
HOWEVER...the nut in many of the plastic spring guides is simply a nut placed into the back of the plastic spring guide. While it may seem like it...the nut does not actually hold the spring guide in place, but rather allows the stock tube to attach to the receiver. The spring guide body itself could (perhaps easily) break free of the nut and then cause havok in the spring area (not good).
BUT....the bodies of some spring guides are actually metal and threaded themselves where the stock tube screw actually screws into the body of the spring guide and then DOES in effect do away with the need for the alignment pins.
So...it depends on what you get. In general...the nut holds the stock tube on...the spring guide tabs/pins keep the spring guide aligned.
I hope that's confusing and muddies things up nicely for you
PS..to the point, replace whatever broken part you have. ALL v2 spring guides will have some sort of nut/threading in the back of it.