Being always the proponent of tools in workshops, and hence the antagonist of Kohaku, I feel I have to chip in once again to the argument.
Tools shouldn't break, I have never broken a decent tool, but then again I do take care of my tools.
Tools do need maintenance though. If I have a chisel, I need to sharpen it from time to time, but I probably won't break it (I do have a broken chisel that I got from someone else, but I just ground in a new edge so that it still works).
However, if a klutz comes in and tries using my tools, there's a good chance that there will be breakages, snapping hacksaw blades comes to mind here. Also, if the tools are bad quality, they can break (drill bits are a good example). Yet more tools will break if you don't maintain them, but that's mostly machines rather than hand tools.
As such, you can't just have something with a certain chance of breaking, it depends on multiple factors, and I think that this should be factored in. If we were to have tools, they would have a base quality level, and then one that goes down over time, and can be raised back up with maintenance. Dwarves would naturally go for the highest quality tools, and when repairing tools they would find the lowest quality tools.
On to the subject of actually having tools. I don't having a generic tool kit is a good idea, it just adds another thing that you require without adding depth. If anything, it's just an annoyance, since you just end up having to make a pile of tool kits every once in a while.
Instead, I think that there should be separate tools, but none of them should be compulsory. An example is building a table, if you want a basic table that holds things, you can cobble one together with a hand saw, bits of 2x4 and some nails. If you want a quality table that looks nice and would sell for more than a fiver, you need chisels, sand paper and planers, to name a few tools. This can be replicated in DF, when you first make a fortress and you want to make tables for your dining room, you can make them with simply an axe.
I think that this would require a revised manager system to work properly, since you would have to be able to do things like ordering sets of items (so you can order a "carpenters kit", which would include chisels, saws, planes etc.).
I think what this discussion boils down into is a conflict of philosophies, I believe that the game should be as deep and intricate as possible, simulating real life essentially (which is why I am against vampires etc), while people like Kohaku believe that the game should be more "gamey". I think that there should be dozens of different tools for different cultures etc, which I can then see moving through the fortress and having an effect, I want there to be an effect when there is a crappy tool maker, I like seeing the causes and interactions of events, like if a cook makes a bad batch and seeing the food poisoning sweep the fortress.