How engineers think:
Many years ago I was talking about job problems with a production engineer at Ford. His claim to fame was the assembly line (Late 1960's) couldn't figure out how to install the front turn signals on a particular new model coming down the line.
His solution was to grab a broomstick and use it to pop the assembly into place after dangling it in the right spot. That's how all of those models went together. However there was no way of un-popping them to change a bulb............ Not his problem.
1970 Oldsmobile backup lights: Step 1 remove rear bumper.
It only matters that it goes down the assembly line easily. What happens afterward, doesn't matter. Someone else's problem.
Lots and lots of auto engines are fully assembled to the front subframe off the main assembly line and then the whole front subframe goes into the vehicle as a single unit with engine and transmission and front suspension and steering already on the subframe. Changing the front antiroll bar on a VW Golf/Jetta Mk5 requires dropping the subframe. Lots of transverse V6 engines have poor access to the back three spark plugs. The timing chain on a VW VR6 engine is on the back of the engine (on the flywheel end) which means the transmission has to come out in order to get to it. They did this because you're never supposed to have to service it. Oops.
There are certain jobs on Ford F250/F350 diesel trucks that require lifting the body off the frame. If it's a pickup truck, that's mildly miserable (at least the cab is separate from the box). If it's an Excursion, the whole body has to come off.
I hate working on cars.
Back to bikes. On Yamaha FZRs (which I'm quite familiar with http://www.gtamotorcycle.com/vbforum/showthread.php?178050-FZR400-winter-overhaul ) there are 2 bolts securing the crankcases together which can only be accessed by disassembling the top end. Cylinders have to come off. If you need to fix something that is entirely in the bottom end (e.g. transmission) you have to do the whole thing.
Spark plug access on a gen 1 zx10r (and I have one of those, too) requires removing the front upper engine mounts. And removing the throttle bodies. And moving the radiator forward slightly to get enough clearance. That means removing seats, fuel tank, belly pan, side fairings, upper fairing, airbox (which is a nuisance on its own), a heat shield around the engine which is another real nuisance, and some emission control hoses. It is more or less an 8 hour job.
In contrast, changing the spark plug on a cbr125 requires unplugging the spark plug cap, and then removing the spark plug. Nothing else. And there is only one of them.