What difference does the range of gear size matter?
Its not about higher and lower end bikes, its about the nature of the bikes and the purposes they are used for and probably some history and snobbery too (-:

You can pay a truly staggering amount of money for a mountain bike (or a tourer or a recumbent) with a triple chainset and equally you can get something that bears a passing resemblance to a road racing bike with a double chainset for a relatively small amount of money (though its a bit harder than finding something cheap that resembles a mountain bike).

You can't ignore weight either - because the bikes with doubles on tend to be road racing bikes and for those weight is significant - as is complexity, removing a front ring (only having two) reduces the chances of you being in the wrong gear or having to make an overly complicated change at a critical moment - however it reduces the range of gears and, if you've watch pro cyclists struggling with genuinely steep hills (e.g. in the UK) you'll know that this can be an issue - although seldom in the Tour de France where the hills are long rather than particularly steep.

Look at city bikes and - assuming they use a derailleur - you'll sometimes see a single chainring at the front and again possibly on a bike that costs more than you might expect.

I've not answered the question so...

The gear range for a bike is substantially dictated by the lowest gear - you start with what you need to deal with the ascents you will be tackling (based on your personal abilities and other factors) and then cover a range dictated by your gearing 'til you get to a top gear. You hope that you end up with something that allows you to complete the climbs whilst not spinning out (getting to the point where you can't pedal fast enough) too early on the descents or whilst on the level. Ideally you'd like the ratios available between the two ends to be reasonably evenly spaced and easily selectable. 

Mountain bikes tackle more extreme slopes so need lower gears - hence want triples. Touring bikes tackle the slopes with more weight, so need lower gears - hence want triples. On a recumbent you want to spin up hills in a low gear but are even more keen on being able to turn a high gear - hence want triples (and sometimes more to extend the gear range). On a "road" bike you're assumed to be fitter and stronger and to pedal with a higher cadence - you also want less weight so you give up on the extra chainring.