Well, I was thinking about it and to be honest found few reasons why these days people are making/playing sucha guitars like 8-9 strings. On the other hand I personally find 9 string as a total exaggeration but as I mentioned aboved there`re few reasons:
1) Nowadays everything tends to be intense, intensive and totally in-your-face giving more experiences to people at short amount of time. It applies to movies (compare eg. horror movies from 80s and modern ones), TV advertisements and it also applies to music as well. That`s why these days we can find extreme styles as dub step and all super uber low tuned metal, nu metal, core styles.
In modern 'metal' music guitar doesn`t plays as important role as it used to lets say 15-20 years ago. Riffs are not as sharp, flamboyant, complicated, complex and sophisticated. You cannot see much difference in the mix between guitar and bass as you could those 20 years ago. Nowadays riffs are blurry, more focused as background rather than first thing that strikes people ears.
These days thanks to low tuning guitar is more connected with bass than ever. Plus lack of solos proves that idea as well.
Let`s compare some metal from 80`s with modern playing, djent, low tuning ect. Extra strings helps with it a lot.
PLUS Dub step is found being mixed in more music styles even in metal these days and dub step is an extreme sounding style. Look at new Korn feat Skrillex.
2) Second reason is MARKET. People have seen 6,7,8 string guitars so why not move it further and see if that extreme can fulfill someone`s needs and make some money at the same time especially that circumstances mentioned above help a lot.
3) When people can make a good use of those extra strings and actually make music why not? I wouldn`t even go as far as 8 string to be honest but take a look at this guy Fracionado. He plays 8 strings (I know it`s only 8, not 9 :D) but he does brilliant job in my opinion.
http://www.youtube.com/watch?v=zpi0w3wPE3MSo why not? Guitars are nearly an ancient instruments, for most people (including myself) they will stay the same for another 10000 years without going crazy with 10 strings and coffee making machines built-in, but if some people fancy going that route, let them do it :D