Just like the previous poster stated, secondaries are important for EVERY position. Especially at the higher levels. The sooner one learns this the more quickly they're ahead of 90% of the people playing this game.
That guide you linked was written in 2010 and people have adjusted their thinking considerably since that time.
As you also mentioned, creating good all-around skills is hard to achieve but that's exactly why those players are so valuable. These days, 24-25 year olds with Hall of Fame/All-time-great potential go unwanted (and sometimes retire) on the transfer wire because their trainer/owner pumped up the primaries, ignored the secondaries completely and nobody playing in the higher divisions wants a high salaried player with terrible secondaries.