I've tried something similar to these rules multiple times in a current Dragonlance campaign I'm running.
Why do it, instead of running another PC?
Because you're dealing with basically a monster stat block - an NPC hireling who fits on a 3x5 card. A single at-will power, and a mix of (say) four simple (no sustain or complex effect) encounter and/or daily powers. No feats. No equipment lists. No rituals. Abbreviated skill list.
That's a lot easier to manage than another section of the table covered in power cards, and splitting the magic items 7 ways to make sure someone isn't falling behind the "numbers curve", and so forth.
Personally, I like how its worked for me in the past and I'm really looking forward to these rules. I had players going through DL7 (Dragons of Light), which is absolutely bulging at the seams with NPC companions. At one stage, every player at the table had a hireling to manage (Theo the Gnome, Vanderjack, Silvara, Theros Ironfeld, etc). And yet, it was surprisingly streamlined. They each had just enough flavor to feel unique (Vanderjack's shifty mercenary tactics, Theros's all-or-nothing hammer swings), yet they took very little extra time to play. My players would agonize over their primary PC's actions, then choose something fairly obvious for the NPC... largely because they didn't have the feats (Power Attack or no Power Attack?) or the powers (sustain my Spiritual Weapon?) where complex decision-making is required.
To add some additional incentive for not treating the NPC as simple cannon fodder, the players also know that their quest XP is tied to how many NPCs survive the session. I've seen players risk their PC's lives to save an NPC (or spend ludicrous amounts of effort to see them healed/raised), simply based on their attachment to that 3x5 card, and their "responsibility" for "keeping it safe".