The benefits of the Great Wall are not really open to debate.
It was insanely effective in a strategic sense- stopping the raids was the whole point to it, and it discouraged numerous invasions as well because it made the Chinese a 'hard' target.
Many people look at a section of wall, note that it could be broken, and assume it was ineffective. But the wall itself was never meant to stand alone- it served as a staging point for the fortress-towns and watchtowers that dotted it's length, and in that sense it was extremely effective from a
strategic standpoint- it effectively allowed the Chinese to stage forward bases and seize territory that would have otherwise been in dispute since even if they seized a section the nomad tactics didn't allow them to use the wall in a military sense. Even if the wall was circumnavigated at one point, it removed much of the nomadic advantage of mobility- if they breached the wall at one point, the only assured safe route of retreat was through the section they had breached- meaning that the massing Chinese forces knew where to go to intercept their retreat.
China was conquered by barbarians twice in history, which leads some to think the wall was ineffective- once during the Song dynasty, whose borders never even reached the Great Wall and thus rendered them unable to use it as an effective deterrent, and once by the Manchus, who were allowed to bring a huge nomadic army into China with the notion they could serve as mercenaries. Both were sort of 'idiot' moves, more the fault of rulers than the wall.
The real benefit of the Great Wall, however, is that it served as such an effective deterrent
to raids that it incentivized neighboring nomadic tribes to
commit to trade rather than simply raid the Chinese countryside. This naturally lead the Chinese to bring in innovations from further away, negotiate with the tribes on a level of equivalent prestige, and play them off against each other in way most sedentary civilizations simply never could.
I honestly doubt that we'll get the chance for a 'Great Wall' equivalent project, but we should go for it if we can. As stated, it's just not meant to stand alone and it's a
strategic defense, not a tactical one.
