Originally Posted by
supton
The overlap can be nice feature. Could you imagine going through all 6 gears in the small chainring, then having to upshift to the big chainring and having to downshift all the way on the rear for the next gear ratio? Ie, zero overlap. That would be very annoying, let's say the terrain was alternating between those two gears. You'd be shifting both derailers constantly.
I can not only imagine it, I once actually built a bike that way. I don't remember for sure what the front chainrings were 28/38/48 maybe but the 5-speed freewheel was 14/18. The concept was to run through the freewheel in one chainring and then make a big double shift all across the corn cob and do it again.
It was a PITA to ride. Imagine cranking up a hill, running out of freewheel cogs and trying to make that double shift. It was impossible to maintain any amount of momentum. Overlap is good because it gives you options as to when to make the front chainring shift.