Preordain is an essential card for the Gush engine, but the more you diversify your draw engine, the less essential it becomes. In a traditional Gush deck where Gush and the restricted spells are your only sources of card advantage, Preordain helps you chain these together in the late game at the cost of a small amount of tempo. If you look at Sylvan Mentor, the deck augments Gush with Planeswalkers and Sylvan Library, which are permanent sources of card advantage/selection. Esper Mentor has Dark Confidants. Some Jeskai builds have JVP. These cards are also threats and/or answers in themselves so you have a card with multiple purposes which means the card selection aspect of Preordain is less needed...you don't need to be digging to find Mentor to end the game. The difference in these approaches is that the other cards are more expensive than Preordain so you need to fill out the manabase more with Moxen, which is @diophan's point. It also makes the tempo bump a bit more significant as you get competition between slamming a Bob/Library/JVP and firing off a Preordain.
Gitaxian Probe has been controversial - Kevin and Steve put forth an entire podcast on the card and in general felt it wasn't necessary. I have the opposite opinion and consider it to be one of the best Blue cards in Vintage. I know Brian feels similarly and I think Rich and Ryan also do, but the rationale is a bit more than can be conveyed in forum post.