In order to answer your question of "why?" I'm going to have to address several assumptions you makes, all of which indicate you have never played a gig with an 8x10 bass cabinet:
Of course they're not very practical to transport, etc, but even with all of that stuff aside, why would someone need it.
This is almost always the first "gem" of advice hauled out (pun intended) regarding 8x10s and it is patently false. I've played several different cab configurations over the years, and the 8x10 was by far the easiest transport. Two 4x10's means more trips and you're more likely to be carrying them by yourself. Then there's skinned knuckles on doorframes from side handles and the extra factor of structural instability. On a lot of wobbly stages, you can take the head off the top of the cabs, but you rarely have space to place two side-by-side (and doing so negates most of their acoustical benefit anwyay). So, if you have need of 8x10 volume, you're no better off carrying smaller cabs just for the mythical 'ease of transport' factor.
Loudness? You can get 4x10 cabs that handle 1000watts RMS.
So what? Power is not a direct indicator of volume, much less power handling.
My dad is a soundman, he was doing a very loud rock show in a venue of about 2500. Bassist was playing through the ampeg fridge with an SVT-2 or similar on top. Mid-song my dad says "here, I'll take the bass out of the PA entirely". Can't even hear the difference. He says, "yeah, and the guy wanted TWO 8x10's". Isn't that what PA is for?
...and that's the very reason the cabs were needed. If in a 2500 seat venue there is no bass in the PA, then the player had better have it onstage. The fact that it could be removed entirely from the PA and still sound the same means that your dad wasn't doing his job. I'll bet the kick drum was thunderous, though.
Plus, the "wash" that envelops the stage, getting into all of the mikes and mudding up everything.
The vertical arrangement of an 8x10 means that it beams its sound in a far more focused manner and actually produces far LESS wash than a shorter cab.
If you can't hear, get a better monitor mix, right?
I have yet to EVER hear a monitor mix that could even come close to doing my bass sound as well as an 8x10 cab. You spoke earlier about stage wash; monitors are FAR, FAR worse for this, especially with bass, than almost any dediciated bass cab. Look at it logically: you have a bass cab on the backline, roughly 10-15 feet from a mic or you have a monitor either in front of of literally right behind a mic. You start pumping anything below 200 Hz through that montior and I guarantee you'll get stage wash a LOT quicker than if the same signal was put through a bass cab on the backline.
I guess it's just an image thing?
There's no need to insult players who use 8x10's, which is what that statement is. Quite simply, it's like making fun of a farmer for driving a tractor. An 8x10 is a tool that lots of bass players have employed to great success for decades regardless of any "image thing." Like tubes, if there was a way to accurately replicate its benefits in anything more "practical,' then it would exist only for nostalgia factor, but the fact remains that it is the de-facto standard for over 90% of touring bass players and except for the super rich mega-PA tours, they're NOT for looks. Just because you think you can get by with something else doesn't mean that everyone can.
) at a loud rock gig, dont knock it 
