Handling, cockpit layout, view, engine sound...many reasons.Problem is, if you get into builds like that, you have to ask yourself why you're not building an FdL or a Python 2 instead.
A P2, in particular, can have (almost) twice the shield and hull, more firepower and a similar jump-range.
I'd also wonder about power and weapons with a Mandy' combat build.
4x C2 and 2x C1 hardpoints are okay but they're not great... and, after you've loaded the ship up with SBs, GSBs and other fighty parts, how much power is left for weapons?
Personally I refuse to fly a FDL. Hate the off center seat, hate the cockpit, hate the view, hate the handling. For me it's the worst ship in ED, detest the FDL.
Python Mk2, now that's a beast. Still has the off center seat though...I have a frag mk2, which makes soloing wing assassination missions far too easy. Due to it being so over powered I got bored of using it very quickly.
For PvP is will potentially make a difference, I get that, but for PvE and open play you only need your dps, shields and hull to be above a certain threshold to make it a very viable combat build.