Garza is still young, but he isn't going to get any better and he isn't good enough to be a number 1.
You don't trade your top prospects for a middle of the rotation starter unless you are just that pitcher away from being a contender.
They traded 2 of their top 3 prospects in the deal, but lets look a little closer shall we. Chris Archer went from basically off the radar to the 27 prospect. He is still struggling with his control (4.7 BB/9 and 12 WP) and he is getting hit hard. He might figure it out still since he is only 22, but this might be a case where Jim Hendry has actually sold high on a guy *gasp*.
Hak-ju Lee is going to be a loss, but lets be clear this guy is still three years away. The Cubs actually have a fair bit of talent in the low minors, and this is actually trading from a position strength (middle infield prospects) for an area of weakness (starting pitching).
I understand your guys point about not contending this year and even next year. But Matt Garza can be a crucial part of the rotation for two more years under team control. This isn't the Cubs going out and getting a 30+ something starter that they are paying 15 million a year.
I know the trendy thing is to say that the Cubs are doomed to be a bad team for many years, and I don't hold out a ton of hope for next season. However, there are lots of available impact free agents in the next couple of seasons, and some big time prospects are coming through the system at this point. Having Matt Garza makes this a better team, and the cost at worst was Hak-ju Lee and a guy that might be slightly better than Matt Garza.