Uninstalled if not needed, but programs just sitting on the disk do not affect performance (assuming adequate disk space) - so primarily, I did mean stopped from starting with Windows.
By uninstalled I hope you mean just stopped from auto starting?
I tried to verify if Avira Free checks for spyware - which is really where Windows Defender comes in. But this would suggest Avira Free doesn't do much of anything! ??? I cannot find on Avira's site at all that says Avira Free checks for spyware, which I think is odd. If Avira does not check for spyware, then WD may be appropriate (if MBAM is not in real-time).
But that said, If Avira does not check for spyware, then I might suggest uninstalling Avira and move to an anti-malware solution that does. If you select Microsoft Security Essentials (MSE), it will disable WD for you, and hopefully the one MSE will consume fewer resources than Avira and WD together.