I have to say, that in general I disagree with this - Although, I would say that more work could be done, allowing predefined colours for moods and some clever stuff like that!!
However, my main reason for disagreeing with this is because this is actually done by design. The point of Windows Phone, and the Accent Colour is that it allows a theme to be applied to the whole phone, not just the native apps within the OS. This means that for me (as a developer) I need to think a little bit more about being able to include this Accent Colour throughout my app, meaning that the user will feel like they have been thought about and that the app belongs on the platform.
I believe that a lot of thought has gone into the allowed colours for accents and these colours have been chosen because they do not clash when put onto either of the dark or light themes. To give an example, the HTC 8x custom accent actually starts to blend in a bit and become harder to read.
There is a big argument for allowing the user to have choice - But the majority of users won't realise that an app looks bad because of their poor choice of colour - They will just blame it on the app or the OS.
At least this way, when a colour is chosen, it works, and it works throughout everything.
So, I hope that most of you will read this and see this question with a slightly different viewpoint - I really don't believe this is lack of 'want' from Microsoft, I actually think it has been thought about very carefully