Warning: Personal opinion ahead!
My workflow is usually:
* Search whether a bugreport for my problem exists already
* If yes, subscribe
* If no, create one
If you do the search via the global top-bar search field, this workflow becomes not straight forward. You have to go back to the homepage, enter Maniphest and click //create task// in the upper right corner. Alternatively you can click the plus icon, but that not perceived as part of the "main page" by many people (after all, it appears in the header, between the user-page and the user-settings icons, next to the logout button), so they tend to ignore it.
My suggestion would be to move the plus icon into the grey area, but still let it appear on every page.