I do hope they can hold on to the tournament. The tournament could give the country a boost and a really positive influx. Also, the whole of Africa could benefit from this football-wise, but for RSA itself it could be a general boost that goes way beyond football alone.
Of course FIFA can do little about security issues in the cities, but in terms of help on infrastructure etc I think FIFA should help out a bit to assure the stadiums are ready in time. FIFA gave their promise to South Africa and should have looked at every possible aspect of the candidacy before making a decision. If they make the decision, they should now stick with it IMO and try to help out South Africa with their organisation if needed. The only reason that'd justify a move would be if security is really out of control, but unless you are in a war zone I don't see FIFA making such a drastic decision to move the tournament.