Btw. I wanted to actually write morph-target library a long time ago and I'm actually really thinking about that again ... thanks
Consider:
1. Base mesh.
2. Eye raise.
3. Jaw Open.
If you simply 'lerp' between 2 and 3, you'll have a half open Jaw, and a half raised eyebrow.
Let's say I want a "REALLY" open jaw, in most software I can extend the weight to 2.0 to achieve this. If you were simply using LERP, the head would scale up 1.5 times it's original size.
Both of these kinda indicate that using LERP is a no-no. A better idea is to subtract the target from the base mesh, and then sum the weighted offsets onto the base mesh.