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 anyway 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.