In a previous life, I did a lot of building and collecting of home audio, where no one would be caught dead using a class D amp for anything, save maybe a subwoofer for a home theatre.

basically, Class D amps are designed to deliver massive power cheaply, with less associated heat, at the expense of SQ -- so it's understandable why they'd be useful for auto and motorcycle applications.

But, what's say you don't need massive power -- like, you're only doing a 2-channel set up? For example, why should I spend $800 for a Class D Focal Sport Amp, when I can get 150w/ch If I use, for example, a class AB JL Audio 300/4 amp in bridged mode? Is there an overwhelming heat issue to worry about? a greater risk of failure? Is the amp too heavy? too big?

I'm trying to figure this all out.

 

Most bikes at most only have a 50 amp max charging system with a smaller capacity battery that will not keep up with with the high demand of power that an A/B class amp.

Even if you ran multiple batteries with an A/B amp the charging system on a bike would be working extremely hard to try and keep up with the electrical demand.
That is my basic explanation from my simple way of putting it.

 

Pretty much what the above said.
Class A/B amplifiers sound great but are large and in-efficient.
Class D amplifiers sound just ok, but are small and more efficient.

Its a trade off.

Cheers