Here is what some common Human Interface Guidelines have to say about when to use them. Most are quite restrictive:
Gnome HIG
An assistant is a secondary window that guides the user through an operation by breaking it into sequential steps. Assistants are useful for making complex operations less intimidating, as they restrict the information visible to the user at any given moment.
[...]
Assistants do have major downsides. After using an assistant it is often hard to figure out where the individual settings aggregated into the assistant are stored. Often people will resort to re-running the assistant, re-entering many settings that they don't want to change.
Assistants are often used in situations where a better solution would be to simplify, or even better automate, the process. Before using an assistant to step people through a complex operation, consider if the operation can be fundamentally simplified so an assistant is unnecessary.
Microsoft Windows Experience Interaction Guidelines:
Consider lightweight alternatives first, such as dialog boxes, task panes, or single pages. Wizards are a heavy UI, best used for multi-step, infrequently performed task. You don't have to use wizards—you can provide helpful information and assistance in any UI.
Apple Human Interface Guidelines
For products with complex setup procedures, a setup assistant can be helpful
(Assistants are not mentioned in any other context, as in the other HIG:s, so I assume that means that Apple think they have no place except for setup)