I've used my TI as just a kayak, man what a great glide...
It goes forever with just a little propulsion! But it can be a bit of a chore getting turned around in tight quarters.
It is also great as a single-outrigger kayak. Just attach one of the amas, then have fun. I did this a few times on one trip, makes for a wonderfully stable platform - I didn't have to worry much (almost at all, certainly not compared to a regular kayak) about my balance while photographing the wildlife. I had a haka along as well so lots of storage / work space in easy reach beside me. I was in a lot of skinny water - maybe 8" deep - so had to take the Mirage drive out, that's when I found it was a lot simpler to just use a single-blade (canoe) paddle so I don't bang against the akas.
That shallow-water paddling is where I really like the TI over my Outback - it tracks SO much straighter. First, I can leave the rudder down but uncleated and it just pushes back if it hits the bottom, the Outback twist-n-stow rudder goes sideways and I find I'm in a hard left turn. With no rudder at all the TI with single ama still tracks quite straight whereas my Outback keeps trying to do donuts and requires lots of course correction.
I haven't had much luck with passengers in a tandem kayak... Maybe with an outrigger I'd be okay! One of my just-a-kayak trips I had a relative along on her first kayak trip. It didn't take long before I suggested she just relax and enjoy the ride, PLEASE stop pedaling!
I have no idea what she was doing but every few strokes there was a strong lunge/rock that I occasionally though was going to dump us over! I also tried a tandem pedal-drive kayak at a demo day with my dad - and didn't like his style either. He pedaled just fine but every so often shifted his weight side-to-side - very suddenly. Going along smoothly then a HUGE lean to one side and back to normal! What the heck...?!?
Now, passengers when SAILING is a blast!
I've had four others (including several small kids) out with me a couple of times. We didn't have much wind so it wasn't exciting but they liked it. Taken one other passenger on several occasions in higher winds and really had fun.
I'll admit I don't sail the TI nearly as much as I take a regular kayak out. I can have my Outback on the water and gone in 5-10 minutes, the TI requires (at best) 20 to (more normally) 30 minutes on either side for setup/launch and recovery/teardown. If I intend to be out for a good long time the TI is worth the extra setup/teardown but most of my outings tend to be just a couple hours in the evening and it isn't worth the effort.