I'm using the Buffer Technologies Mag-Cinch for my AR mags.  Problem is, you have to place the mags at different heights so that the right (lower) mag clears the ejection port cover (dust cover).  This makes the combined mags longer than necessary.  I prefer to put the mags at the same height (so they fit nicely in my pouches), and I removed the port cover so the right mag will clear.

The question is:  Is the ejection port cover really necessary?  I've carried an AR/M16 for the last 18 years, and I've never had much use for the cover.  Am I asking for trouble without it?  Thx.

If the use of the Mag-Cinched mags is more important than keeping your chamber clean, then by all means, lose the dust cover.

I personally consider it DAMNED important, myself.  My first intro to the dust cover's utility occurred in basic training at Ft. Dix, NJ in 1977.  We were running from one range to another, preparatory to qualifying.  It had been raining and we were going down a hill covered with red clay mud.  I had my rifle (XM16E1, refurbished to M16A1 specs) at port arms, and some left handed guy ran by me and his front sight/barrel snagged my sling and spun me around like a top.  I slid several yards down the hill on my chest with the rifle still underneath me.  Upon getting up, I was a red clay mud monster with a diagonal green stripe where the rifle had been. Luckily I had a mag in it and the dust cover closed.  I was able to clean the mud away from the sights, mag and ejection port area and qualified a few minutes later with no malfunctions. I would hate to have done something like that without a dust cover (closed) on my rifle.  If all your shooting is done in clean environments, it's probably not important, but if not... I vote for the dust cover, myself.  It should be ingrained as a habit to close the dustcover each time you handle the rifle, so as to make it automatic to keep it closed, IMHO.
Hope helpful,
John

Since you have elected to use the Mag-Cinch then I would assume you're setting your AR up for a combat type situation. If that is the case, keep the ejection port cover. If you use your rifle for target practice, shooting from the bench, etc., then it doesn't really matter if you have the cover or not. I have a DPMS upper, floated, fluted bull-barrel stainless 16" for varmint/target shooting that does not have a port cover or a way to attach one.