decompression

[DRE]

That's a very good point you made there - not only does the nitrogen takes up space that otherwise could be used by the O2, it also doesn't optimize the oxygen window, ie the pressure differential between the partial pressure of arterial inert gas (nitrogen and/or helium) and the partial pressure of inspired O2. If you consider the oxygen window to be at 100% at 20ft on pure O2, doing the math for EAN 80 @ 30ft, which will roughly give you the same PPO2 as O2 @ 20ft, will give you a less than optimal oxygen window, hence you're doing inefficient deco. This I consider the main reason why not to use EAN80 for deco purposes. All of the arguments why you should use it (such as it's safer to mix, you can get more in a tank, it's safer because maintaining proper boyancy sin't as critical, etc.) can all be reduced to poor diving skills (if you can't maintain proper boyancy @ 20ft you shoudn't be tech diving in the first place - in fact, you should redo your OW class and get it right).
Also, nitrogen is probably the worst gas to deco from - it damages your red blood cells when inspired at higher PP's (deep air diving is as stupid as drunk driving in that regard), it cannot flow through cell walls, so your chances of subclinical DCS inrease exponentially, so why would you put in a deco gas in the first place? - especially when O2 is so readily available and you don't have to deal with the mixing nightmares (what's easier than topping off O2 with O2???).
Helium on the other hand has all of the posiitve aspects for deco that N2 doesn't have - so it's definitely the gas to use (case in point: DSAT - Padis's tech branch, or what's in a name - just launchd their trimix program).
I welcome anyone who has any arguments pro using EAN 80 to join this discussion - maybe we can get started having a real debate then.