While I agree on the fact that the fuel is used to lubricate and cool the fuel pump, I contend that the pump always draws from the bottom of the tank so the contaminants are not a factor in running the fuel down low. That is why you have a fuel filter. They are always there and are sucked up (the sock catches the big chunks) even with a full tank. Now sloshing of a fuller tank may spread the distribution of the contaminants but they are still there in a flat bottomed tank.
Actually, if you look at where the pick-up is on most fuel tanks, it's a little bit
ABOVE the bottom of the tank - the opening generally clears the bottom by
at least 1/2 an inch or so, even when the pick-up is in a 'corral' or some sort of lowered section of the tank base; and while there's a fair bit of gas left in the tank, even given that there's
some movement and sloshing around of the gas remaining in the tank, while ever there's a fair bit of gas remaining, the scungy, manky stuff suspended in the fluid and any liquid contaminants unable to be dissolved in the gas (like water, condensation, et al) all tends to stay down below that slightly raised pick-up, so that the gas generally pretty much shows a distinct 'line of separation' between the gas that's being sucked up by the fuel system and that watery, contaminated stuff that you really don't want to be sucked into your fuel system. If you think about this, it's actually a pretty nifty piece of design - simply by raising the fuel pick-up a bit ABOVE the bottom of the tank, the larger volume of gas in your tank separates out the manky stuff and doesn't suck it up into the fuel system - normally!
BUT, once the level of gas left in the tank starts getting down a fair bit, the sloshing around of what little 'good' gas is left in the tank tends to become waaayyy more active and agitated than it was when the level was up above about 1/4 of a tank full even with the gentlest of riding/driving!! A full tank of gas usually has very little 'free surface effect', but that sloshing increases exponentially as the level drops!
And that tends to stir up all the scungy, manky, contaminated stuff that basically remained settled down below the pick-up tube while the gas tank was closer to full, stirring it in with all the good stuff that you
DO want your engine to run on more and more the closer you get to empty, and it's
THAT stirring, agitating, and mixing in of all the manky, contaminated stuff that happens once the remaining fuel level gets low that basically
FORCES your fuel system to suck in all the nasties in a way that it doesn't normally!!
Add to that the fact that the larger volume of gas in a tank with more than 1/4 of its capacity remaining can fairly easily disperse all the heat generated by the fuel pump etc (and I'm pretty sure that most of you'd be surprised at how much that heat generated actually is!
) but once the fuel remaining level starts to get low enough, not only does the act of riding/driving start mixing in all the scungy stuff, but since most of the 'contaminant' type stuff is also usually watery, what little fuel is left fairly quickly starts to heat up and have a much harder time keeping all the important bits of your fuel system cool and lubricated! By design, this is not usually a major issue while the gas in the tank level is anything closer to full than about 1/4, but once the level of gas remaining drops much below that, it starts to rapidly become a problem,
a REAL problem! And that is why most vehicle manufacturers 'glovebox manuals' usually have an admonition in them somewhere that basically tells you
to NEVER run your gas tank dry, or to always keep at least 1/4 of the indicated tank full of gas in the tank, &/or to fill up before your gas tank gets much lower!! 
Just Sayin'... Again!
Ps: Most manufacturers who install gas gauges these days, BRP included, do tend to build this 'necessary fuel remaining' level into their gas gauge display and their low fuel warning light triggers in the hope that most drivers out there will heed the gauge &/or warning and top up before the remaining fuel level gets too low to cause too much immediate damage - they don't really seem to mind a whole lot about the low level damage that might occur (especially early in the piece) if you run just a little low, I guess cos while that damage, once started, is irreversible and simply gets worse over time, they're well aware that it usually won't become a significant performance degradation &/or a major concern until after their warranty runs out!!
So they're generally happy enough if you're silly enough to run the level too low
occasionally, or just a bit, but you might be surprised about the records they keep if you ever let on that you ran it absolutely dry, especially in the first few weeks of ownership, or if one of your on-board computers rats you out! And just between you'n me, those bloody sneaky computers
DO do that!
That 'running dry' thing is something they
really want recorded, and if you happen to get sprung having done that, it can be the reason all sorts of potentially expensive warranty claims are either refused outright, or at least vehemently argued that it was your actions that caused the issue!
