My wife's Land Rover also requires 90 or better. It's a 4 litre very low reaving engine. One would wonder why in hell they'd require you to run 90 Oct for this snail on wheels. I've ran 87,88,89,91, and 92 in and have never noticed one bit of difference in it's performance. But it seems that this particular vehicle has a problem with carbon build up on the exhaust valves. The company knew about this, and figured the higher octane gas would burn better, thus less carbon build up on the valves. Running it harder (not babying it like tea-tottlers do) will also have the desired effect of not having carbon build up on the exhaust valves.
So when the car makers suggest you put x octane or higher in your car, I'd just suck it up and do it. They made the car, hence they probably know what they're talking about.
87 300E (140K - Like new)
97 Land Rover Disc. SE7 (Wife)