BTX has it's merits for sure, but I think it was a definite push from Intel towards alleviating heat stress on components from their power hungry, heat inducing CPU's. BTX is an attempt to correct those flaws, re-enforcing standardisation for component design and placement. By standardising the motherboard topology you also gain a bit more control of extraneous costs in terms of accounting for design variations. Everything becomes standardised and, technically at least, that means everything is less expensive to build (more than likely not for PC enthusiasts such as us though). Not to mention that with only one fan, your PC should be quieter as well. :yumyum:
Imho ATX has already undergone numerous transitions and evolutions becoming slightly more BTX each time (for example using heatpipe tech instead of traditional fans on chipset controllers etc). I think more peeps will adopt BTX once the lines between the two become considerably more blurred.