I'll echo what Kat said, that it isn't necessarily a custom system that slows down developers, but the way in which that custom system is written. One of the things I've been doing over on the development server is to make a set of systems which are very easy to work with, both in terms of developing new features and deploying existing ones. Wrapping up commonly used or complex functionality in an easy to use system can actually improve development, making these feature easy to re-use and reducing the number of places at which a bug can occur. There is an ongoing effort to improve the current systems and make them much easier to work with.
The problem with the default systems is that you either have to take the whole thing or none of it at all. It can be awkward to build extra functionality on the top of this, whereas with a custom system you can change and tweak every minute detail. I have all the details needed to make a clone of the default systems. I'd much rather see a custom system that mimicked default than actually using default in it's raw form, purely because a custom system allows for more flexible development.
Just to reiterate again that this thread is absolutely no promise that a raw form of default will be introduced, and it would be unwise for players to draw false conclusions. I do not want to see anyone disappointed because they thought we were bringing default back, when instead we decide not to.
