Its a good thing to have; helps the aero, keeps the muck out of the engine bay and gives some protection when you hit a speed humpup etc. so keep it.
That said, it is a complete pain, its made of cream cheese and the fittings are no better. Don't worry about using cable ties, they are better than the original fittings. Replace the bolts along the front edge and the self tappers if you can. Also worth pop riveting the ally bit back on to put some stenght back in.
Mine was modified, before I got it, with rectangular hole below the oil filter. It means you can do a proper oil change wihout having to muck about for ages removing and replacing the undertray. 10 mins instead of several hours.
GTV and Racing Driver