In extending a helping hand, you've forged a valuable cooperative relationship. If, on the initial round of the PD with a new partner, your partner accepts your aid but does not return it, then you'll know not to cooperate with him in the future. Tit for tat' is not as effective when there is some chance that you will misread your partner's actions as a sell-out when he, in fact, meant to cooperate. If you give 'tit for tat' from the very first sell out, then you might opt out of what might have been a mutually beneficial relationship. When there is some chance of misinterpretation, it's best to be a bit more forgiving.
Social environments are very noisy.
As anyone over the age of 15 knows, communication can be very difficult. In noisy social enironments, your best bet is to be pretty darn forgiving. Extending faith, hope, love, and mercy, to your partners in the repeated prisoner's dilemma of real world survival is an effective survival strategy. Of course, at some point, it becomes obvious that someone is indeed habitually exploitative. Even accounting for noise, it is clear that he's just not going to hold up his end of the social bargin.