There is a clear idea behind the recommendation. This is one of the fundamentals that are inherent in Go and although some people like Gu Li and Sedol among others, are known for their fighting style, it does not mean that they neglect their groups and try desperately to make a base for that group while being chased around by the other player. Look at joseki for example, a great many joseki are concerned with building bases.
Anyway the logic of defending before attacking is grounded in insight. If you invade, but leave a big weakness in your position, the invasion will become a bad move if the opponent is strong enough. You see, while the invasion is being handled, the weakness could come in handy for the opponent, maybe even making miai of attacking the invasion and breaking apart territory using the weakness. Also, if you have 1 weak group and invade, making another, you immediately set yourself up for a splitting attack and in the end it is likely that you must give up one of your groups.
I remember reading something in Attack and defense about how even pros known for they aggressive style, defend regularly.
Defense can be a powerful attack. If you remove one of your weaknesses, the opponents weaknesses still remain.
(Just fyi making a shimari and playing kakari is usually thought of as the same in value regarding 3-4/5-3/5-4. 4-4 might be the same, dunno.)
But something like making a base for your stones in important areas aren't usually missed by strong players. There is a style where you leave your positions unfinished and create alot of confusion and start big fights, this is true. But if you don't know how to handle the situation properly, it is better to make a safe group and attack from solid positions. It is extremely hard to attack an opponent if you have weak groups all over the board. Also, sometimes a defense might be nessecary to ensure that you keep a large territory that you've spent many moves creating. Don't let your positions fall apart with one move from the opponent, but prevent them from falling apart in the first place.
If you get split, attacked and pushed around due to not defending at an appropriate moment in time, then you will most likely lose the game. Defense is actually a strong resistance. If your opponent can't touch your positions you're free to operate where you please.
Anyway I will stop trying to explain now, people much better than me will most likely give a shorter, more thorough and easy to understand explanation soon
