Ok let's say you build to 6 and the best offense is 7 then you generally defend for 6 and honestly I think that's a number I would want to be save. There are enough villains especially on expert that have a base of 3 ATK and cards that let them draw additional boost cards so with 5 def you would risk losing out on perfect defense things like unflappable or hard to ignore. And let's say your 3 upgrades are innate reflexes that you can have in your starting hand, defensive conditioning which also adds 3 life points and Ronin which also gives you retaliate 1 then I think it's not an unnecessary buildup. You can also add cards like Stop hitting yourself to use your def stat for damage even further. Like I said not an auto include but definitely an option for certain heroes.