There's (unsurprisingly) a lot of talk about how the DRS dealt with the LBW referral for Ian Bell, where umpire Billy Bowden chose to stick with his not out decision after the tracking system "predicted" the ball was going to hit the stumps. I'm going to take my own stride down the pitch on this topic - you judge whether I'm out, stumped or LBW!
The way the DRS works, the decision is ultimately up to the umpire, no matter where the point of impact is, but according to the regulations, if the point of impact has been less than 2.5m, the third umpire would simply have told him it was in line, hitting the stumps and so on, and I can't imagine that being ignored. However, in the over 2.5 m situation, the umpire not only given more details concerning distances and wherhe the ball is projected to go, but is told to use "normal crticketing principles" in deciding whether to overrule his original decision.
Cricinfo says that the reason for the 2.5m distinction is because that's where the predictions become less reliable. Of course the reliability of the trajectory prediction does depend on factors including distance from the stumps, and it's easy to believe that this was a factor in the minds of some of the people involved in accepting the guidelines. However, while the Hawkeye reports of MCC testing don't in my opinion rigorously address this issue, they also give reasonably different criteria for their "extreme LBW" classification, hinting (along with the dependence on the original decision being "not out") that this isn't what the 2.5m is really about. In fact, Paul Hawkins says the main reason for it is to ensure that the traditional dispensation for batsmen coming down the pitch "continues to be the case", even when the benefit is no longer founded in quite as much doubt.
The relevant document form Hawkeye is found online, and was first brought to my attention by Kartikeya Date. He has used the traditional approach to LBWs as a reason to oppose the use of technology for LBWs, and while he argues against the DRS even in this form on several grounds, I gather that he thinks it is better to include this clause than not.
In some ways, the 2.5m rule seems odd, but it is one of the more logical of the current systems oddities. The basic intention is that if either "traditional cricketing principles" applied to the trajectory or the projection itself say not out, it's not out. Of course, it's less transparent than simply using the calculated uncertainties of the systems, or even deciding on mathematical factors to simulate the traditional approach. Bell, Watson (v Zimbabwe) and Paine (v England in Perth) have all been given not out in a >2.5m situation where the computer said the ball was hitting the stumps. One decision was upheld by the umpire, two were overturned. We can't know whether this is because different factors were involved in the original decisions, the umpires have different ideas of what "traditional cricketing principles" are, or simply are differently inclined to overturn there own decisions. The first possible reason is in line with the aims of the clause, the second arguably so and the third clearly an unwelcome human factor. Describing the process more explicitly might help (if such a thing can be agreed on), but applying it to "out" decisions as well misses the point.
Personally, assuming the demonstration of the tracking and projection accuracy was more satisfying than indicated by the Hawkeye document, I would like to put more emphasis on the predictions of the tracking systems, but not on the grounds of transparency or lack of human involvement. Actually, even without using the tracking systems in real time, I'm happy to see umpires let them inform future decisions to some extent, as has already reportedly happened. I would rather make LBW decisions as literally as possible than maintain traditional levels of doubt in the process, but that's not because of a technological argument, but because I think I'd like the change it brings.
I'm certainly not pretending it wouldn't be a change. Which leaves us with the current clause in the DRS, intended to avoid a drastic change to interpretation of the LBW law. It might not be perfect in that regard, given that it depends on how the umpires use it, but criticism needs to either have the same intention, or tell us why it doesn't matter.