Why did the keystone not reveal the location of the Grail immediately? It's not like the riddles that come with the keystone would have kept the "unworthy" from progressing along the path to the Grail. They could be solved by anyone, no matter their true intent. Sure, it might have taken a few days at most for a dedicated grail hunter to solve those riddles, but once the keystone is in the wrong person's hands, it is already too late, so why make the process slightly longer by giving them unescessary riddles? If someone is able to acquire the keystone by getting hold of the bank key and the bank number, they have had obvious access to the sénéchaux, and the person with the keystone has either gained their trust, or been resourceful enough to get a hold of it without it. Any 2 day riddles that follow is like wrapping a valuable object in tin foil after placing it in a safe - unescessary.
Alas, I think the only reason for those riddles was so they may be used as a plot point: to lengthen the characters' adventures. Saunière's love of riddles should and would have been kept seperate from the enormously important task he was entrusted with. He needed to keep the grail secret. I am sure the Priory of Sion's policy regarding the keystone's instructions would have been to keep them concise - since they never know how much time the holder of that keystone would have in case of an emergency. Those riddles would actually endanger the righteous people who would have needed immediate access to the grail for protection or whatever other reason. In short, I thought Brown could have at least dedicated a paragraph or two to construct a valid reason for the necessity of the riddles in the keystone.