While digital synths sound good now (and have low latency) you still need a UI that is suited for making music (a good midi controller). Since you already need a DSP to hook that stuff up to a USB port it's not a stretch to just up the computing power a bit and also run your synth software on the device. Also: while a generic computer has more power, you need a dedicated machine to not run into software issues (especially music software on mac often doesn't survive upgrades). An integrated synth just makes more sense to me.