A user posted an answer to my question. I found the answer useful, although it didn't answer the particular question well. Being a disciplined user, he deleted his own answer. The problem is now I want to go back and re-read his answer but it's gone. Is there any way to recover this data?
You can't do it yourself, but users with more than 10k reputation on the target site can.
I assume this is the answer you're looking for. I don't see anything here that the answerer would be bothered by reposting, so here you are:
In this case I would say clearly performance reasons where the deciding factor - the SimpleBitVector32 struct (and its cousin BitVector32) represent a single integer and allow modification of it through helper methods.
The overhead of using a class instead would would kill performance, since you would need an additional pointer into the heap. Using a struct it only needs 4 byte.
If this struct was immutable you couldn't change the integer it represents - so each modification method would have to return a new SimpleBitVector32, which again is a performance problem.