The game is quite “Invade and Conquer”. How do one square that off with a player winning because (s)he has built a wonder but is losing in the field.
Adapt your strategy. A good player is able to adapt to any situation. How would you explain the benefits of strong defensive civs if there was no way win by being defensive.
Some players are good in the open field as you said , other are better at economy and some are good at turtle-ing. And that sort of makes a rock paper scissors situation. For example a player good in the open field of battle will out maneuver a great economy by constantly harassing and out-valuing his opponent, but will fall flat against a strong wall and a few well placed defensive towers.
But even a strong stone will fall at a constant battering of rain drop-like assaults from a good economy that is able to sustain the siege.
So, to be honest i do not mind the wonder idea, i do like it as i don’t know of any other RTS that promotes a defense as a strong wining strategy.
It might also be because i tent to be a strong macro player that loves to swarm the map and win due to sheer numbers, so i rarely have problems with super defensive plays, but i do hate it when i get harassed … i can never seem to be able to cope with that 
@BornPants34 said:
Adapt your strategy. A good player is able to adapt to any situation.
How would you explain the benefits of strong defensive civs if there was no way win by being defensive.
Exactly.
It’s just like in sports matches.
The best team does not always win the match.
It is tactics, insight, training and, of course, a little luck.