My friends who have the OG Epic (I have two with that phone) do not have any intermittent problems using the nightly builds. My wife who has a SGSIII, which is plauged with plenty of other issues, doesn't have issues with MMS either; she is also running CM10.1 nightlies. Do you have a reference for this information? A trouble ticket, perhaps? Again, I appreciate the information and will utilize that to fix my issue.
I had something from the cyanogenmod github with conversations between beblos mkasick, wtogami (original epiccm team) on the issue when I was working to address it for myself. I can't find it anymore. Things have gotten even more confusing now with APNs for LTE and no-LTE...
I had the OG epic and had issues (and my wife did as well where it would work great and then stop on the evo running cm9) for a while until the epiccm changed the APNs used in cm9 to the ones attached (and I changed my wife's phone). You will notice in the attachment that none of the sprint ones are using the proxy as that is from cm9. If you look at the ones on your phone from 10.1 (and go.cm's OG epic), they are using a proxy which is not accessible if you are not attached to the 3g network. The mms.apk or whatever is attempting to use the proxy and is hanging is my assumption. I have not looked at the code. but I can tell from my router that it attempted to connect to 68.28.31.7 while on wifi and even after disconnecting wifi didn't render it 'fixed' once it happened.
mmsproxy="68.28.31.7" mmsport="80" (i.e. http://68.28.31.7:80)
All of this is why people tell you to 'delete' the telephony.db to 'fix' the issue because when you do that, it re-reads the apn.conf file and may select the right one and may work again for some time.
Summary: I don't care if you try it. If you would like to, great. I agree it would be wonderful if CM fixed it. But like I said before, It really isn't a BUILD issue. It is an intermittent upstream issue.