Why would anyone go and buy a separate DAC to use with the ADMs? That's like buying a Benchmark DAC1 Pre, plugging in an Audiolab M-DAC and then complaining that there are not enough analogue connections for a TT and a CD player. Pointless argument to say the least.
The speakers have two optical and one analogue input, how they are used would be down to the owner and what equipment they had, but at any rate, I'd imagine the intended use to be for someone with a couple of digital sources and one analogue. It's a bit short sighted to assume that they are 'computer speakers'. Perhaps if the interest increases in the future, USB might be re-instigated, but that rather depends on demand.
And your last point (more of the first really) How many sources do most people have? I'd say that the majority of users needs would be catered for.