I just tryed an experiment.
I have Asio4all on a laptop I use at work which I only work with midi files on in Home Studio.
I brought my Scarlett 6i6 in today and insstalled the Focusrite driver.
I booted Sonar and selected all the 6i6 Boxes and unchecked asio4all. It work without issue.
I also played videos and Media player all played via the Scarlett.
So I re booted computer, re opened Sonar , still using the 6i6.
I turned the 6i6 off and Sonar let me re connect to the Asio4all, it worked. I turned the Scarlett back on, Sonar asked if I'd like to use this device , yes, unchecked asio4all and re checked all the 6i6 boxed , hit OK and it worked.
So everything I tried shows there is no conflict with using Asio4all alongside (at least) my Focusrite ASIO drivers.
I think there where issues in the past but somehow I think asio4all is now improved and we need to re look at it in a different way.
Of note is I tried to use asio4all with the Scarlett 6i6 and it only gave me 2 inputs , channel 3/4 and the SPDIF and the MIDI where all missing. So it certainly sucks as a driver replacement for a real audio interface but it does seem the best option when there is no ASIO driver available. WASAPI was also failed to give me all my in/out as well as it refused to work in 44.1. It wanted only 48.
Anyhow, there should be no reason to uninstall asio4all from the OP computer. But he should choose the M Audio driver, unless,,, I've have bad experience with Maudio drivers in the past and so there might be a huge difference between my 6i6 and the M Audio ASIO driver quality.