The purpose of friendly fire is to let people with bad aim realize that they have to aim better. If you have a good team it makes no difference and if you have a bad team it makes no difference. There are admins and kick votes for the intentional tkers.
With that in mind, I came up with an idea for friendly fire I intend to test out. The motivation behind this idea is that I don't like dretch punting, but don't want to lose an evolution point every time a dretch gets in the way.
The idea is basically that you lose however many credits or ep the teammate you killed was worth (i.e., how much they spent to get there), with the exceptions that you only lose a fraction of that if you only did a fraction of the damage, and builders within a certain distance from their overmind or reactor cost something.
Of course, when someone with a bsuit+chaingun walks in front of your fully charged luci, losing 80% or so of his health, you'll still lose a load of credits. (But slashing a tyrant once will not cost all that much.)
I think that the current team kill thing (not tks, but kills counting toward the next stage) is messed up, so I might see about making the whole credits/points for death thing saner (and hopefully cleaner).