The problems with friction drive are blown WAY out of proportion by people that haven't tried it.
I'm personally a big fan. If you look at my other posts you'll see my Kollmorgen friction drive setup (although the pictures of the bike are old, still the same basic idea). I'm running 36 volts at the moment and it's good for around 22 mph. Today I got 15 miles out of my sla batteries (three 7ah).
It's true. Friction drive sucks in the rain. I don't see any way around that. But in the dry it's great. My bike is also a great hill climber and I live in an area with some good size hills (I'm also not a small guy). I don't know the grades but they are a lot steeper than a wheelchair ramp. I also ride some easy dirt trails in my area without a problem. The big issue for me is the weight of the batteries. But with all the money I saved on my 'ghetto' setup (heck, I even call it that) I can spring for some pings soon.
I have a theory on efficiency when it comes to friction drive too. I think it's better than many other methods. Think about it. If you try to accelerate too fast with a hub motor you just dump amps from your battery and don't move. If you do the same with friction drive it will slip and you know to back off thus saving battery power. I've also read some theories that all of your power is wasted through the 'friction' between the roller and the tire. Bull I say!
You don't need to have the roller THAT tight on the tire. In fact, I keep mine fairly loose. I also haven't had any problems with tire wear in well over 1000 miles (and I've slipped the roller plenty). I suppose if you had a TON of power going through a VERY abrasive roller you could put a wear spot on a tire but with any reasonable motor (for a bike anyway) it shouldn't be an issue. I also don't see a need to have the ability to adjust the tension while riding. If you have a clutch bearing inside the roller there's no need to remove it from the tire. It feels like a normal bike when the motor is off. Btw, it's also very quiet.
I'm using a knurled 6061-t6 aluminum roller and I've found it wears more than the tire. I think what is does after a while is push the knurls over a bit (I'm using a smaller knurling tool). I just put it back in my lathe and re-knurl it. The diameter of it, however, hasn't changed since it was made.
I started out thinking friction drive would be a fun thing to play with until I moved up to a hub motor but now I just don't see a reason to switch.
My next project will be an rc motor mounted to some u-channel aluminum with, you guessed it, friction drive. I'm hoping to make a very light setup for short trips. Heck, Recumpence even said he has a friend with an rc motor friction setup that will go 48 mph
I know most people here like exotic setups and the latest stuff (I do too) but for me (and my uses) it's the easiest low cost way to get a stealthy e-bike.