One thing you're missing is maintenance. There's a difference between reinventing a wheel because that's what you set out to do, and doing it due to false laziness. When it's not specifically this wheel you're trying to construct, but an entire car, you should take some well tested wheel and reduce your own amount of work and maintenance (incl bugs and all that goes along).

If your goal is to build a dynamic website, then use one of the existing templating systems. If your goal is to write a templating system, then (well, duh) write your own.

Some wheels are not very round; many have flaws. Reinventing wheels is good, as long as you're doing it consciously. Obliviously fumbling around for not knowing better will just dramatically increase the amount of lead in your foot.

Makeshifts last the longest.