I'm not sure what your scoring system is, but in my league a RB get 6 points for rushing/receiving TD's and 2 points for every 20 yards.
That said, Tiki hasn't scored many TD's for me this year, but he has been VERY consistant and a great back-up RB for me. He gets me 50-60 rushing yards and 50-60 receiving yards every game for 10-12 points a game.
He's no Preist Holmes or Rickey Williams, but you can book 10-12 points each and every game from him. Looking at your lineup, you'd use him as a back-up anyways.
If I had to drop one player for him, I'd think about Antwain Smith, Lamar Gordon or if you have a 5th WR that you rarely use.
Smith was great last year, but has seemed to be inconsitant this year. In my league, Smith has scored 4, 6, 20, 8, and 8 points for a total of 46 points in 5 games. Tiki's had 12, 10, 10, 14, 12, and 8 for 66 points in 6 games.
Again, your scoring my be different and Smith is the way to go if that's the case.
Hope this LONG message helps : )