What units per em do you tend to use during fontdevelopment?

The interpolate nodes tool was built in to FLS5. I used it more than any other feature. I've talked with Yuri and Adam about having it re-implemented in FL7.

You can achieve some similar functionality in FL7 using X-servant and Y-servant node properties to indicate nodes that should interpolate when shifting other nodes, but this is not as convenient as the FLS5 interpolate nodes tool, in which the behaviour of the individual nodes was constrained within the particular tool session, and within that session one could touch any node and determine whether it interpolated or not, reset individual nodes, repeat movements, define relative or absolute movements, etc.. After the tool session, you didn't need to remember which nodes had servant properties assigned and think about whether you wanted those properties to be retained or if you should reset them to regular nodes.

Basically, it is the difference between deciding ahead of time how you are going to move things and figuring out which nodes you want to be interpolated, and making those decisions during the moving process.