wingnut, your going to get a 50:50 split on this one. Its something that has been discussed by many overlanders over time. Its going to come down to personal pref at the end of the day. As pikey has rightly pointed out, it does not take much to fold a defender cab.
you mention NGO LC but you fail to remember your defender is going to be heavily loaded with kit and is therefore not a good comparison to standard spec LC.
roll overs are actually easier to achieve than some people realise. we are not talking about dakar style dune smashing. imagine a heavily loaded vehicle driven by you after 5 days of piste driving 200km offroad everyday. tired, hot want to get to the destination of the day. the sun is high and you a) hit a small ridge at 30mph or b) go off a small drop into an oued at 30mph. either of which you didnt see from lack of shadows (high sun, easily done). should you not keep it on its wheels and it goes over (either direction, dont matter) then you could potentially be stuffed depending on the situation/location. trapped in the cab with an unconcious passenger at your side a day from anybody passing you? not something i would want with my girlfriend travelling with me.
If you had a cage fitted and little damage done, you could right the vehicle and be on your way again to the nearest garage to check it out. it could mean the difference between carrying on or turning around and flying home.
seeing as your getting a garage to prep your rover for you, if you intend to keep it then it must be worth the little extra cost.
other advantages. external cages mean you can mount a rack/tent and not stress the roof gutters. limits body panel damage if rolled. mount kit externally (sand ladders, spade)
disadvantages: extra weight, some of which will be high up. cost. internal cage will eat into your load space.
thats about all i can think of right now. make sure the tubing is well speced before parting with your £££. where did the garage say they would get the roll cage from?
G