First, defense has very little value so far in terms of saving resources, may be more for keeping medals. We argued with dbuk once what will save you more gold max defense or max caravans. I still believe that max caravans win. Secondly, in that minimal defense value, walls have minimal effect and it will be even lower with time. Like in real life those walls became useless. And the last is huge value of walls starting from lvl10 and low hp increase. It saves you from wallbreakers better, but not everyone is using it. So it is a mostly a decorative element showing that you may a) cheat b) invest money like a fool c) actively play france exploiting a bug which builts units 90% faster on training blessing, instead of 20% faster