NGEN is eating up all my disk space

For a couple months now I've woken up every couple days to find my C:\ drive out of space. After some digging I found my C:\Windows\Temp folder was full of 3MB log files -- ~40GB worth, numbering in the 10s of thousands. Reading through them they seem to have something to do with Visual Studio, but they're cryptic, long, and I don't know what I'm looking for.

(I do some development, so Visual Studio isn't a surprise. I have both 2012 Pro and 2013 Community installed.)

Today it finally sunk in that most of these files are being created early morning, around 4:30AM, so I checked my scheduled tasks and found two tasks, both of which failed around 4:34AM:

.NET Framework NGEN v4.0.30319

.NET Framework NGEN v4.0.30319 64

with error message

Task Scheduler failed to start instance "{eae34cb6-c300-4811-8394-c15d557a59c5}" of "\Microsoft\Windows.NET Framework.NET Framework NGEN v4.0.30319" task for user "NT AUTHORITY\SYSTEM" . Additional Data: Error Value: 2147946720.

and Task Scheduler says

The operator or administrator has refused the request. (0x800710E0)

I cannot edit either task and there are no triggers set that specify when they should run. I am able to disable them, though.

I know NGEN is the .NET Native Image Generator and has to do with background compiling assemblies, but I don't know much more than that.

So my questions are: Should I disable these? If not, why not? What can I do about them filling up my temp folder on a regular basis?

PC is dual-core Athlon 3.1GHz 64-bit, 6GB ram, Windows 8.1 Pro, system drive is 120GB, normally 40GB free.

Here's a sample log file.