Djokovic's place is assured by his slam count. I find the 'all round game' point a bit of a red herring in this context,
it's 'post hoc ergo propter hoc' logic. Guys win more slams win them because they can do more stuff really well, have an all round game, if we want to put it that way.
But lets imagine a scenario in which a player wins multiple slams-lets say 6, but does so because of a massive serve and booming backhand, his forehand is s**t, he dies if he goes to the net, his movement isn't great, he's always injured and he's known for being a choker. Yet he wins 6 slams.
Lets compare this hypothetical player with Djoko, they have comparable weeks at no and about the same no of tournaments etc.
Why should our estimation of those players be swayed by Djoko's all round game verses our hypothetical player, lets call him 'Sid's, abilities? They achieve more or less the same, but they simply had different approaches. The more negative you are about Sids shortcomings, the more positive you must be about his assets, since that given his weaknesses his strengths must be all the more phenomenal.
It seems to me that Sid and Djoko are on par, they're both great because of what the achieved, not how aesthetically pleasing or otherwise, how sexy or otherwise, how tenacious or otherwise, or a million other things.