Yes. Only if you need it. One way to need it is if you want the range. You can go as high as standard tuning and as low as well... F#. Rawr.
The other reason, which is my reason, is that you basically have three guitars in one. Right now I have my ESP tuned to Drop C, and I kinda miss playing in standard. But I also really want to play in B tuning. And I also want to play in F#. I could get a 6 string and tune it standard, and two seven strings, one standard and one down to F#, or, I can spend all that money on one really nice 8 string and get all the tunings I need.