Usually when we think of modals, we think of dialog boxes like the one below from Google Documents. Aza’s critique applies to this kind of modal. After you call up the find/replace box, you can’t click anywhere but inside the dialog. That means you can’t scroll the document underneath the dialog or copy and paste a word from the document into the dialog box while the dialog box is displayed.
Registrations are annoying. People hate them. No matter how clear benefits of registration are and no matter how much of them WILL register eventually, they still hate it. But, in the other hand, they want to finish their jobs online, and most of them are willing to provide some information in order to finish their jobs. So why don't make their lives easier?