Curious: What's the affair with "one big knob"-style interface?

I don't record bands or drums so two audio channels are plenty for me and normally overkill as I typically record only one audio channel at a time.  Every now and then I'll record an acoustic guitar in stereo, electric using direct input and through an amplifier or keyboard with stereo output but not very often.
 
Rarely, I will record line outputs of a mixer as a favor for a church friend or high school teacher but always their microphones and mixer.

I understand the concept of only ever needing to record one or two inputs at a time. I'm in the same boat myself. I am never going to record a full band or even multiple musicians in a single take in my home studio. Never. One or two mic inputs is plenty for me. 


But when I upgraded my interface last year, I bought an RME UFX with 30 ins and 30 outs and I still feel it was the best choice for me. Because its not about needing to record live mics, its about having tons of extra ins and outs for whatever outboard gear strikes my fancy. 
 
Plugins are great and have lots of utility. But IMO, there isn't an 1176 plug out there that truly measures up to what the hardware can do. You can get more plugin reverbs than you can shake a stick at and lots of them sound fabulous. But none of them sound quite the same as the old Lexicon hardware units. And while those old Lexicons can sound a bit dated today, its still a sound that I like and use.
 
Now I can already hear all the folks out there saying 'I don't use outboard gear and I never will'. My response to them is simply this. Two years ago, I said the exact same thing. Never will I use any outboard processing or other gear in my studio. Never. 
 
Then I stumbled onto a great deal on an old lexicon and since I had those digital I/O's that I wasn't using on my interface anyway, what could it hurt. Then I decided to build one of those 1176 kits. That first kit led to a second and I've already have all the electronic parts for a third. That's how it happens and it can happen to anyone. 
 
So I'm in the same boat with OP, I don't really understand the obsession with the one-knob devices. I like having options and a one-knob seems like the opposite of that.

You need as many inputs as you have input instruments and devices you use. 
I like to keep each of my inputs set up and ready to roll. I don't want to have to re patch and set levels. So if your interface only has 2 inputs, but you play 3 instruments and sing you really need 4. 
I returned to using my old Tascam us1641 and man is that working better than my 6i6. I just leave everything plugged in. And I have track templates for each channel and instrument. Grab the Mandolin, insert the mandolin track template and go. 
The 6i6 works OK too, but then I have to use those SPDIF inputs via my Yamaha 01v.  Much nicer to have all those channels. 
The main reason I would buy a 2 channel interface would only be because it had real good pre amps.. but that's just as easy to do with other hardware pre amps. 

From the manufacturer's viewpoint simpler the hardware is cheaper to manufacture. From the user's viewpoint simpler hardware means fewer points of failure. Personally, I do not bother much with the hardware since I can set up everything using software. It would not matter if the device only came with I/O ports.

Well there can certainly be better quality as you climb the ladder. It's that good old diminishing return factor. Like converters, how much better are the converters and the pre amps on the higher end devices. lets just say they are 10 % better for the sake of this topic.  
For a hobby studio do you really need that 10% better quality? For a weekend band recording your shows? 
Those users will look for more in/outs at the best price and for the most part will never notice any quality was lost.
 
Round trip latency is another important factor that you pay more for better performance. Once again, RTL doesn't come into play with the way many of us work.  
 

Kev999
I only "need" 2 ins and 4 outs, but having a few extra ones is always handy and saves a bit of swapping around. It's convenient to leave some things permanently connected if you use them a lot.

Exactly.  Having dedicated inputs with levels set and ready to go is really nice.