1) Jesse knows Walts 'formula'. There aren't that many ways to cook.
2) Amphetamines aren't difficult to manufacture. Especially if you have laboratory grade equipment.
3) Gustavo told Walt that he would provide expert help. If he already has access to these experts, then why would it be so important to him to employ a high school teacher?
If Jesse (an uneducated junkie) is capable of making almost as good a product as Walt, then surely anyone with a reasonable grasp of chemistry would be able to do the same - wouldn't you think?
Couldn't they basically hire anyone with a science degree?
It doesn't make any sense to me.
They're going to an enormous amount of effort to keep him on as a cook and he's a huge risk, given Skylar, Hank and the Mexican twins.
As the plot goes on, it's becoming too complicated to keep it believable.
Basically for the show to exist, Walt needs to keep cooking. Now he has no motivation to do so, so they create a flimsy series of events and compromise his character severely.