As it was happening I thought the broken down truck was a trap for Rick and Daryl - Jesus had met up with a few others and staged the flat tire to draw them in. Jesus somehow wound up on the roof of the truck 5 seconds after being bound with a rope. I'm not sure what the rationale is behind Rick wanting to recruit more people. Other than having another person for manual labor, the risks of bringing a stranger into the group would seldom be offset by any potential gain - unless the person was a doctor or had some other expertise that was in dire need. It's another mouth to feed and a security risk. There isn't enough guns/ammo to arm everyone, if I remember correctly, so you do gain a body but any fighting they do on your behalf would have to be with a knife or improvised weapon.
Especially in light of the whole "you were right -- no, YOU were right!" conversation with Daryl in the truck, I think the "rationale" is the whole "if we lose touch with what makes us humans different from zombies, then the zombies have already won" meme, and the resulting tension with the "we protect our own at all costs" thing. Basically, it's another version of the the same supposed thematic tension that 24 kept trying to mine -- "do we let these strangers in or just kill them as the most expedient way to eliminate a potential threat" is TWD's version of "do I illegally torture this guy to make him tell me where the bomb is hidden?"