At the end of the day, this is all form over substance. However, I for one am very much in favor of the change Jerry has described.
First, knock out races happen to horses at all levels. A horse who has an established level of 15 who jumps up and runs a 7 is a very heavy bounce candidate. You do not need to see such a horse go from +4 to -4 to be able to see it.
Second, there should be no special credence given because some magical fixed number barrier has been broken. In other words, the difference between a -0.5 and +0.5 should be no different than the difference between -2 and -1 or +1 and +2 but people tend to give a special oomph for breaking the magical barrier. The same is true in the Beyer system. Why should the difference between a 101 and a 99 be more significant than the difference between a 97 and a 99 or a 101 and a 103? (I do recognize that as you get faster, one point takes on more and more significance -- in other words, I look at the difference between horses capable of a 1 versus those capable of a 4 as being a far more significant difference than the difference between horses running 31s and 34s).
Third, it is really just a matter of convenience. When you are comparing things, it is just easier to try to filter out all differences and distractions. Can I deal with negative numbers, yes. Would it make things a little easier to have almost all numbers be one or the other, yes. The only problem with switching that I see is that people very experienced in using the numbers have in their heads certain conceptions about what type of numbers are necessary to win a particular type of race and use that conception to determine whether a particular field is weak or strong for its level. This conception can be important, for example, when you are considering how to handle a european import or a first time starter (or if you have a horse that is very hard to get a read on for other reasons). If you know the field is weak, you give the outsider more consideratoin. IF you know the field is strong, you recognize that the outsider needs more to be there. If a 5 point scale adjustment is made, these conceptions will need to go through a readjustment so that people are not thrown off. However, I think it is easier to deal with the momentary adjustment of these conceptions while going through transition than it is to have to forever deal with negatives and positives together all the time.
All this being said, at the end of the day, this is form over substance and nothing more than a matter of convenience. But, if I had to choose a writing system, I would rather go with an alphabet than use characters or kanji, so all things being equal, opt for convenience. I think there is a famous philosophical principle about this -- Occum\'s Razor it might be called if I am remembering correctly.