Wow, good spot! Thanks, but unfortunately I do need this feature with the brush tool, for quick (and smooth) sketching.
Some observations after a quick play around in the AP trial:
There is a slight issue with the 'constrain' implementation with the Pixel Tool: it's too keen to continue the previous line. Starting a stroke and *then* holding shift should just apply the constraint, but currently it will retroactively connect the previous stroke. (This happens up until you move the new stroke, meaning that to use the constraint feature on its own requires a very precise start to the stroke, which is not fun). Photoshop's implementation beats this.
The 'alternate' draw mode on the Pixel tool would work great with the paint brush tool! The Ctrl key isn't currently used for the brush, so why not allow a quick erase?
Damn, this make it even more frustrating that Serif can't quite pull this all together