With reference to the wind debate, the two minute wind is usually given by a FISO, however in some situations it can be fairly different than the instant wind. I had a situation about two months ago where a large CB was growing to the south east of the field. The two minute wind was giving around 280/10 knots and the instant wind was giving 080/06 knots. Obviously, the instant wind was much more accurate than the two minute average so that was given with the prefix "instant wind".
Also, with reference to the origional post, I always explain it by saying to somebody that when we say "take off at your discretion" we are simply saying "take off if you want to" and I then say if somebody offered you a biscuit by saying "take a biscuit if you want to" would you reply with "take a biscuit if I want to" and then leave everybody wondering what you're going to do? Besides, saying "taking off, G-CD" is much easier than saying "take off at my discretion, G-CD." What does "take off at my discretion" even mean anyway?