That''s a good question. I am not 100% sure that I know why sheets are normally spliced to storm jibs and I have asked myself this same question.
On one hand it seems as if it would be important to be able to clear a fouled sheet by untying it at the clew, but on the other hand, I have generally conjectured that there were a number of possible reasons that storm jib sheets are spliced onto the sail. The two most compelling reasons that I come up with is that storm jibs tend to flog extremely violently in the high winds that they are used. Having a piece of hardware or knots flogging violently would greatly increase the risk of damage if the clew of the sail hit something. The other big issue, of course, is the risk that the sheet would come undone. Getting storm sails up and down in the harsh conditions that they are used, means that it becomes all the more imperative that the sheets not come loose from the sail.
What ever you do, storm sail sheets should be securely attached and low stretch line for these critical sheets are very important as well.