Hard one to explain this. I am not aware of any imaging editor that has this feature, but if Serif could add it, it would be a very nice enhancement.
The Issue
Let's imagine you are masking a complex shape in a photo, using your pressure sensitive tablet (although the same issue applies to mouse use BTW).
Say your maxium brush size is 100 pixels, with 50% brush hardness. This means the soft edge on the brush is 25 pixels wide,
but only when the brush is at this this size.
Reduce the brush size to 10 pixels (either by pressure sensitivity, or manually if using a mouse) and your soft edge on the brush is now only 2.5 pixels wide. I.e. your "soft" brush has actually become a very hard brush!
This effect is very visible when working on a complex mask. You scale down your brush to do some fine work, and realise you have just added a bunch of razor-sharp details to the original soft edge. I find myself constantly juggling the brush softness with brush size to compensate.
I envisage a new brush scaling option where you specify a brush softness value in pixels, not percent, and then as the brush scales down from the maximum size the soft edge moves inwards, eventually making the centre of the brush less opaque.
I suspect it sounds more complicated than it is, I drew a little picture to show what I mean, as long as I can persuade the BB software to embed a link to Picasa-
Adding this type of brush scaling would definately improve the process of masking tricky images, and would probably have other uses too.