Mgar, the whole time I read your post I was muttering to myself that if players can lose their marks that's a flaw in the system. Then you went meta. Very nice.
Whether the flaw is inherent or meta I'll take the system that yields fewer goals. I found
this about Benitez employing zonal:
Zonal marking and the attached ‘criticism’ is however, one of those things that only gets highlighted when it goes wrong and therefore gives the perception that it is indeed an unworkable and vulnerable tactic that isn’t to be trusted or relied upon, whether it be the first minute or the 117th minute of the Champions League final. However, as Rafa Benitez rightly points out on his own website (rafabenitez.com), Liverpool were twice best at preventing goals from set pieces in the Premier League during his time and they were consistently inside the top four teams at defending set pieces, with the exception of just one season.
Of course, that's anecdotal and proves nothing. Then there's
this:
The aim of this study was to explore tactical behaviour when defending corner kicks within the English Premier League. Specifically, the types of marking and defensive players positioned at the goalposts were investigated. A total of 436 corner kicks from 50 English Premier League games were analysed. The most commonly used marking system was one-to-one marking (90.1% of total corners), with zonal marking being used less often (9.9% of total corners). There was no significant association between the marking set-up and the number of attempts at goal conceded when defending corner kicks (p>0.05). However, teams who applied zonal marking conceded fewer goals and fewer attempts at goal than teams who used one-to-one marking. The most common set-up for defenders positioned at the goalposts was having a defender positioned only on the far post (47.3% of total corners). There was no significant association between the positioning of defensive players at goalposts and the number of attempts at goal conceded when defending corner kicks. A further detailed analysis of defending corner kicks is still required and suggestions have been made for future studies.
The rest seems to be behind a firewall, and I'm not paying $43. Seems like a small sample since fewer than 50 zonal kicks were analyzed. But if we did take it as accurate, then of interest to me would be that the first article suggests zonal is a stronger counter-attack system while the second article says zonal and man are equally effective defensively. Given that, even if it feels worse from a fan psychology perspective, I'll take the zonal. Of course, I'd still like to see more data.