I suggest you add Timani's tackle stats. They are, well, interesting viewing.
From what I can gather, he played 74 minutes across 4 tests, made 11 tackles and missed 7 for a 61% success rate.
The success rate is low, but it is interesting that he is attempting a tackle every 4.1 minutes on the field (and making one every 6.7 minutes).
Compare that to Simmons who attempted one every 9.7 minutes and made one every 10.8 minutes.
Horwill attempted one every 7.4 minutes and made one every 10.7 minutes.
Fardy attempted one every 5 minutes and made one every 5.6 minutes (amazing stats).
Hooper attempted one every 5.9 minutes and mae one every 7.5 minutes.
Mowen attempted one every 6.4 minutes and made one every 8.2 minutes.
From that I think you could infer that Timani is being sent out there with the aim of trying to smash people as often as possible and in that process is missing plenty of tackles.
Simmons success rate is extremely high but he is attempting tackles infrequently compared to his peers. This would suggest to me that he is very good at making the tackles that come his way but isn't going looking to do extra defensive work.
Any way you slice it, Horwill's stats are terrible both in workload and success rate.
To add in another stat, rugbystats has Timani being the first to the ruck 9 times in his 78 minutes played. I am not going to sum up all the other players but that strikes me as a very high rate for his minutes played.
I in no way think Timani has been excellent when he has come off the bench. I think he has fulfilled a role though. Assuming he is starting this weekend it will be interesting to see how he goes.