The thing about the draft vs free agency vs trades is that the draft is usually the only way to get a player you can build a championship-caliber team around. What happened two years ago, with all the talent that switched teams, is really unusual. Usually, players of that caliber only switch teams at the end of their careers, like Kevin Garnett, or when a player demands a trade, which doesn't happen as often as some people think. Also, by trading for a player you're giving up assets that will take time to replace, using up valuable years of a veteran player's career.
If you look at most of the championship teams over the last 30 years, usually, most of their best players were drafted by that team: the Lakers' Magic, Worthy, Kobe, and Bynum; the Celtics' Bird, McHale, Pierce and Rondo; the Isiah/Dumars Pistons; the Jordan/Pippen/Grant Bulls; Olajuwon's Rockets; the Duncan/Robinson/Manu/Parker Spurs; Wade's Heat; and Nowitzki's Mavs. There's no surefire way to build a championship team, and the draft is always a crapshoot, but it's usually the only way to get one of these players.
Now the joy of my world is in Zion
How beautiful if nothing more
Than to wait at Zion's door
I've never been in love like this before
Now let me pray to keep you from
The perils that will surely come