Deleting items and not having the discount re-calculate was, in fact, an oversight.  If it hasn't been fixed, it should be; I sent the code to Dave to incorporate a couple of weeks ago.

I was having a similar issue where people would add things to the cart until they got a good discount rate, fill out their shipping information, delete items from the cart (without the reduced rate re-calculating), then checking out, effectively ripping me off.  It has since been fixed.