This is all assuming that you have email confirmation enabled.

In some cases you don't want to take the user back to their mailbox to confirm and then get back to the forum. It defeats a great part of the express registration, IMHO.
The back link to the forum is definitely needed on my site. Having the user go up to the menu to click befuddles some of the older generation.

The redirect gets them there and they don't need to view the user name and password which will be emailed to them.
Spam is a problem on numerous big boards. I've got friends who are experiencing a solid number of spammers defeating the captcha. I find that if I change the questions, it minimizes the spam since these spammers must have an smart remember entry to their forum spam. So the captcha can work, it just has to be there and implemented along with SFS.
I still think this is a phenomenal mod and deserving of MOM, which I noted this month.