Perhaps it's a personal quirk, but this has always annoyed me, with almost all brush engines LOL!

In the brush options, why isn't spacing affected by scaling?

I know this may seem obscure to some, but take the chainlink brush for example. If you allow scaling to be affected by pressure, the size of the links will deminish as the pressure eases, but the spacing doesn't seem to allow for the smaller elements, it stays constant.

If the spacing could be linked to the scaling variation, this would give you a 'faux perspective' effect (no overlaping forms, of course). In the example given, the links would seem to recede from the viewer as the pressure eased (or the line shape thinned), this would be a very useful option. Imagine being able to simply lay a path to show a lace pattern flowing around a skirt hem, or a paved road winding off into the distance.

If you can include an option for random spacing, it seems to me not to be too big a stretch to allow an option to link spacing to scaling?

I understand that few would see the benefit of this (I've had this discussion before), so the demand may not be enough to bother adding such an option.