I think this is a great idea as it does cut down on general automated scripts that look for register.php however, I also suggest using something like the StopForumSpam plug in to deal with some of the other spammers or those that figure out that you changed the name of the page.
As far as questions/answers there are those that will go to the trouble of figuring out all of your Q&A and put them into the script to deal with.
Multiple levels of defense are all good, as long as it doesn't compromise the registration of legitimate users.
I hope that 4.1 (or maybe 4.0.3) will get the option to change the register.php name.
StopForumSpam for 4.x:
https://vborg.vbsupport.ru/showthread.php?t=230921