E type FD's are usually only found on very low end bikes or full suspension bikes that don't have a conventional seat tube with a place to mount a FD with a clamp. I suspect they come on low end bikes because the assembly is so easy: the whole thing is installed along with the bottom bracket and crankset. There's no adjustability in terms of angle or up/down, but if you want fast assembly that's a good thing. To be clear, though, there are high end full suspension bikes that, because of their frame design, have to use an E type FD (I can see someone with a $5000 mountain bike chiming in on this). And therefore there are higher end E type FD's available as well.