Personally I think an instant bolt would add some interesting tactical options for spells users (Magician types mostly). Naturally such a bolt would have to be relatively weak in power, but having to choose between an instant shock bolt, say, and a much slower but potentially much more powerful lightning bolt could be interesting.
Actually now that I think about it, why are there only two possible spell speeds inherent in Spell Law, instant and non-instant (caster level being equal) for all spells in the book? Shouldn't casting speed be a variable parameter, like range, target, and duration is?