I have came across numerous cars over the years which have had serious problems using off-road 100 watt bulbs. Burnt out switches, burnt out bulb holders and burnt wiring, burnt out fuse boxes, etc, etc. Now if a wee fire starts your insurers may well smell a rat.
On most modern cars the wiring fitted is just about capable of carrying its prescibed loads without any additional ampage being applied. Also on many later models the lighting is controlled via an UCH so there are added risks.
Oh don't you just love the smell of toasted silicon.:d
Also not only are they strictly illegal but a danger to other road users.