Well, I agree restricting cross class abilities as a class (in other words, without the Soul Crystal equipped) was not a good idea, but like most of the issues with ARR's design, blame the 1.0 players whom SE polled as to what they wanted ARR to be like and what changes they wanted to be implemented in ARR's design - one of the main changes was they wanted designated party roles for each class, and each class to have an actual identity, especially when in a party, hence the restriction on abilities, a restriction which is even tighter when a Job is equipped. SE merely gave players what they wanted. So, gone horribly right I guess?

Also Flash may be a spell but it has an instant cast time anyway, making Swiftcast totally redundant. But I digress.