Perhaps starting with the $500 bike if you don't know what your needs are yet, but once you have that $500 bike, ridden the hell out of it, and now understand your needs better -- doing the jump right to that $1,500 bike if that's what you need.

Chances are, quite a few people will find that the $500 bike fits their needs, they end up not biking as much as they thought they would, and just keep on trucking and being happy with their bike.

Now, if you are someone that already has a clear idea of "I want to ride with the best, win races, have a super light/fast bike, and going to be getting hardcore into biking" --- then I'd agree that you are better off sinking the investment into the better bike right from the start.

But, within reason : you might have a bike that's $1,500 and $3,000 - both meet your needs / goals / etc, and probably the $3,000 is the better bike, but is the performance gains you'll gain out of it worth 2x the price? Some times having a budget in mind can help guide you to a reasonable choice for what you are actually wanting to do.

Or, maybe they can afford $1,500 now - and over the years, swap out / upgrade components on the bike as they get the money for it.