I am not a Dead fan.
I will say that I DO think I'd really like them if I took the time to dig deeply into their catalog.
I don't think your comparison to the Allman Bros., is really appropriate. Two different bands; two different genres; two different styles. The only thing they really have in common is that they're "jam bands."
From the Dead that I've heard (I have listened to some "deep cuts," as one of my closest friends is a Dead Head), I will also opine that your assessment of Jerry Garcia's guitar playing is off the mark. He was a great guitarist. Far above average.
I will also disagree with your "singing is atrocious," comment. Anything I've heard, regardless of who's on lead, has been great. And their harmonies are also great.
I think you're passing judgment on a band that you've really not even listened to.