again that sounds great, but how come still a massive difference between a hybrid (which is faster than a MTB) and the road bike?
Plus what are you going to do, change tyres everytime? Carry knobbly and inner tubes when you park next to off road site? Then change back to road tyres?
Have another set of wheels? Carry them with you on a ride?
It really depends what you are going to use the bike for doesn't it? Imo, an MTB and by that I mean a decent XC MTB (not a downhill jobby), has less compromises in more areas. You can ride it on the road (albeit less efficiently we get that), you can ride it off road, you can throw it down hills off-road, you can beat some single track, you can toddle along with the kids around the park. Fitting some lower profile tyres is the quickest way to make it more 'road-friendly' without a great deal of expense.
A road-bike can only be ridden on the road. (Of course if you only ever ride the bike on the road then get a road bike.)
A hybrid can be ridden on the road, and off-road to a certain extent (albeit less efficiently that an MTB) and it's far less fun off-road.
.... and I haven't mentioned a cyclo-cross bike, which is another do-it-all option with a greater leaning to the road-side.
Edit: Must learn to type quicker, apologies for cross-posts.