I think the point of this practice is that it can cause inconveniences both to ATC and other aircraft, I think if it was unsafe it would have been brought up and changed along time ago.
I experienced this very topic today after landing at LGW, we taxied to our gate and was told to hold while a major US carrier finished its pushback into the cul-de-sac and also wait for it to taxi to the holding point (not very far from its gate) before we could proceed.
After there push back they called ATC to ask if they could hold position because they had not completed their takeoff calculations and bugs. This did not go down well with ATC who was rather busy at the time and was told to taxi to the holding point regardless because right now he was blocking everyone!
Iv never seen an a/c taxi out so slow! I mean even the A380 does not taxi this slow. There were several aircraft waiting one on stand for push, us waiting to get onto stand and the rest were waiting in line for takeoff sequence. They did not show much appreciation to the fact that other a/c were waiting for them.
Now this may have been because they were partly heads down and taxing at the same time in which I can understand why they would taxi so slow, but this was also at the expense of other aircraft. LGW is not the greatest taxi layout but the taxi instructions to the US aircraft was simple enough because the gate was so close to the holding point, so it was not to do with complicated taxi instructions.
9 times out of 10 this practices probably goes unnoticed and generally works without inconveniencing others but I can see why the few times it does can irritate both ATC and other crews.
Its really no big issue, as long as it does not happen all the time, I'm sure ATC will bring in new procedures if it gets out of hand such as the ones imposed in Italy.