we already have a great partial system for showing your worthiness, it just only applies to enemies right now and no one else can see it: credits/evos, humans get a percentage of credits for a kill equal to the portion of damage they did to the enemy, and the alien who did the most damage to a human gets the evo. Plus you lose some for tk's! (actually... this might be a server thing, but at least the code is already there to do it).
I think all the above proposed scoring systems for only the non-build part is great, so the rest of this post is going to be building related
first thought: +points for building/repairing, -points for deconning, although this wouldn't be a very true system, it would have some amount of effectiveness, as normally not all buildings are deconned, some are destroyed (either by enemies or teammates), so a move would yield zero point gain (unfortunately), a good builder who is sticking around repairing and rebuilding destroyed buildings would be able to rack up points as fast as the enemy can take them down
however, after mulling on that, someone could then just build buildings out in the middle of nowhere, they will immediately get destroyed (no point loss for builder), then they build again, rinse and repeat, racking up lots of points for absolutely no benefit for the team
so, why not factor in the time a building lasts? you don't get points for building, and you don't get negative points for deconning. For each x amount of time the building is there you get y points, if you only have one builder who constructs the entire base, they don't get any points for building it, but the longer that base lasts, the more points they get, and if it is immediately destroyed because of bad design they get very few points (maybe even have a small point penalty if it lasts less then say 2 minutes, denoting /horrible/ placement)
the problem with this is that someone could build the entire base, and make it very self-sufficient at that, and some random guy could come in, decon 1 building at a time, replace it with his own, and start gaining all the points for himself
to fix that, maybe have it so if someone builds in the same spot (not exactly the same spot, maybe if the old building and new building's bounding build box thing would have been overlapping) the original builder starts gaining points again for there original building that just got rebuilt by some random dude (however, if the building gets destroyed by an enemy ((NOT TEAMMATE)) and someone else rebuilds the building, the re-builder starts gaining those points, as the original was apparently not placed well enough to keep from being destroyed
another thought: i think damage done by rets/teslas/acids/hives/whatever should be factored in, just not to too large of a degree, otherwise we are back to people building rets/acids where they don't help defend the base at all just to get damage points, the system where you would lose points for short lasting buildings would curb this a bit, but adding points for damage into the equation helps fortify the need for well placed defensive structures
so, since im out of time to type at the moment, and i have exhausted most of my ideas, heres a basic equation for just the build points
PBS (player build score): (A*(Total time all buildings have lasted) + B*(damage done by buildings) + C*(amount repaired) - D*(number of buildings that lasted for less then E minutes))/F