I'd also say that head-fi shouldn't have a ratings system.
1. Putting a numerical value on amps isn't that informative. People have different personal tastes of what sounds good, different values of various features, different tolerances of different types of flaws, and so on. The ratings for books, movies, albums, games, etc. all encounter this type of problem.
2. A numerical rating system implies transitivity, which may not always be the case. If people prefer X to Y and Y to Z, it may not happen that people prefer X to Z. But, this is exactly what a numerical ratings system would enforce.
3. And, of course, there's the impracticality of assigning the actual rating. One possible method is for people to vote. But, is it right, that a rating from someone who has listened and owned the amp for months to be given equal weight to someone who has only heard the amp for a few minutes? And, if not, how does one assign the weight?
4. It would needlessly create tension in the forums. Some people will likely get upset if their preferred amps do not rate well (especially if they have a financial interest!)
Awhile back, I thought it would be really useful to have a sort of relational database where people could input their opinions if they preferred one specific headphone to another specific headphone. Now, I think such a database would not be so useful because it would not take into account people's criteria for judgment.