Yes, it is not a must for goalkeepers, but goalkeepers who have the ability to do so have a plus point in this era of modern football. But again I say that it should not neglect the abilities of a goalkeeper that should be, as in the points I made earlier.
Now what happens to David de Gea is, when he is considered not to have good passing accuracy coupled with their defense that is not better organized. No matter how good a player is, he will not have a good pass when his teammates cannot open up space and can move without the ball.
This is a difficult discussion, but I think that any modern goalkeeper should be able to participate in building up the game. On the highest level I think most of the teams have a goalkeeper who is able to support the team with the ability to initiate high paced attacks. If one team has that and the other teams doesn't, one attack can make the difference between winning and losing. That is why these versatile goalkeepers are among the most expensive players in the market. The value has increased by a lot in the last few years.
Well that's what I mean too, maybe we've also seen how a goalkeeper can end up being the first person to start an attack that leads to a goal, be it with their long pass or through an attack from below.
Like I said earlier that this is a plus for a goalkeper. Not only can they keep a goal from being conceded, they can also start attacks from their feet. But again I would say that it is a plus, or in other words, they should not forget their main responsibility as a goalkeeper.
There are several examples to be found on Youtube for goalkeepers and how they give an assist with either a shot or some of them can throw the ball so precisely and like across the whole field that this can make a difference. But having a goalkeeper who also is close to having the quality to be a field-player is the optimal situation for any team. It can take a lot of pressure from the defensive players when they know that they can pass the ball back without hesitation. Otherwise it can often be seen how critical situations emerge out of nothing because players want to avoid to pass the ball back. Frankly, most goalkeepers today are quite good with their feet and they are usually integrated into the game. But some of them are really mastering their role.