Do I NEED a seven? No. I'm sure I could play a six for the rest of my life and be perfectly happy.
But what fn would that be?
The added low end is one of the perks of seven strings, true, but that's nothing you can't get by detuning. A seven allows you to jump between low riffing and standard-register soloing on the fly, which is a perk, but not essential- you can just write around that. And likewise, while the seven lets you go down an extra fourth in any position, you can compensate for that with position shifts. Same for riffing- if you're willing to jump around, I'm sure you could arrange anything on on a 7 for a 6 (well, almost). And after a year or so of playing 6's exclusively, I probably wouldn't miss it.
So why do I play a 7? Cause they're fuckin' cool and I don't WANT to have to jump around unless I want to. That and I like the wider neck. There's some downsides (jimi style chording is virtually impossible) so I doubt I'll ever sell my last 6 for a 7 (although it could happen, I suppose, I know better than to say never), frankly, I just LIKE playing 7's more than 6's.
The fact it's still seen as a fairly exotic instrument is pretty cool, too.
Lots of people play guitar. Almost no one plays seven-string guitar. So, you'll be talking up some girl at a party and mention you're a seven-string guitarist, and she'll go "wow..."
Ok, ok, that's not the ONLY reason I play them, but it sure helps...
Actually, come to think of it, the reason I first bought my 7620 was the symmetry of it appealed to me- it always bothered me on some level that the guitar didn't have a "center" string.
-D