Whoever agreed that has no idea what they are talking about. Even if that is big friends of yours they have no idea what they are talking about.
It isnt a flashy stat that makes you hurt anyone, but you have to be able to stop damage. That is very important.
Whoring stats is bad. Leaving just 1 stat far behind is not a good idea.
1 little stat means 'anyone' can get past you and exploit it. ie you will be taking a boatload of damage you wouldnt otherwise be taking if you had a bit more defence.
back in the day, a lot of people ignored defense, and it worked.
right now, it's not the best idea to ignore it, but it's not the death sentence you keep saying it is.
plenty people have 1 low stat and can still do their job in wars.
while his build is far from ideal, leaving one stat behind may mean faster training if done properly (not for SSL users like you, but for the 99%), and that stat can catch up later once you start using SEs for the highest one.
whoring a single stat is also the fastest way to reach high totals with SEs. Split 100SEs across 4 stats at 1.5b and you wont even get to 2b on each, but focus them on a single stat and you're way stronger that if you'd split them.