The point of self implemented allocators is not, that they are all better than malloc or that the implementor is more wise or that there is some magical additional stuff or even that some code is saved (the oposite is true) ...
The point is only, that malloc implements the most general case (what it must, since it is the magical "I can do it all" tool (at least in memory management)). When you go away from the general case you can find several specific cases, that can be implemented specialized for this case -- and those specific cases could be implemented very much more efficient (concerning speed, not coding!).
That is the whole point. Also there are some specific allocators that also implement some safeguides for common memory problems. But that seems to be more of the exception, when I take into account the feedback.