It is a weakness in his game but I'm not sure it's a weakness in our game. If we play a high line we have rapid defenders who can cover the ground between them and DDG faster than any striker can. I think that's better than DDG charging out and risking getting caught in no man's land.
It's not a 'weakness' but there's a liability there as long balls over the top with speedy strikers (basically most teams) puts pressure on our defenders and team as it pegs us back. Facing our own goal, our defenders can't turn and pass it back to the midfield and seeing DDG so reluctant, they're not gonna pass to him. Next option? Try and shield it out for a goal kick, throw in or boot it out. Result? Our team has to drop 10-30 yards to defend in our own half due to a simple long ball. DDG coming to collect half of those balls would help so much.
The thing is, I remember at least two times, where it would have been VERY comfortable/sensible balls for DDG to collect, even if he lightly sprinted to the penalty box D. Instead, Jones or Bailly had to play it out under pressure, who don't forget were often isolated in that situation, so they had to hold onto it or if/when back up came. All very unneccesary.
In the grand scheme of things, obviously there's more to look at e.g if we play a high line then why are we not pressing those players, who play the long balls? However, DDG has to take some accountability here.
It's a glaring weakness imo (to me/fans who watch every minute) because he has so few but because he/we can get away with it, doesn't mean he/we shouldn't try to improve on it though. In a way I understand Mourinho's tactics/coaching as he's trying to simplify things for the players (post LVG) and because it's effective. He tells Smalling not to 'worry' about passing (or ask more) because he knows he's poor at it, he doesn't ask for offensive output from players like Herrera and Darmian because it's not what he expects from them etc.