i think your missing the point, maybe watchtowers make bad defenders better and save them from giving away 100's of vills, but you could defend without losing vills before watchtowers existed. i have, others have, both before and after watchtowers.
now if you were arguing watchtowers should be built if they cost no resources and took no farm space, id be hard pressed to state why you shouldnt, as in even if they are useless like i say,, if thiers no down side then why not. but this isnt the case. its takes away troop space.
In the past defending was purely tactical decision making, its not luck or a hunch per sey, its analysing the opp
now when you consider from my perspective, i am going to be opped in 2 weeks, im pretty sure my skillset allows me defend without major village loss without having any watchtowers built, however i have 10 villages in prime locations for building watchtowers that happen to be empty.
if i build 10 watchtowers, i gain nothing as i could already defend the opp
if i build 10 def vills i can reduce loss's all round or assist a teammate
if i build 10 off vills i have more attacking potential.
i could pose a case for the latter 2 options, but why would i opt for the 1st one.
in late game ive played against people far larger than me, people who have mountains of pp to spam, people with 5 manning a account, these a solid advantages and ive faired just fine, i believe whatever advantage watchtowers give, if they do indeed give a competent player one, to be negligible in comparison to size, pp wealth, activity abuse. but as i say, thinking about it they probably do help bad players fair better at defending.