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Pro Ed is the standard text editor
As the standard text editor, it's available on multiple systems.
Pro Ed has a consistent user interface and error reportage
Con Not a lot of features
While extremely fast and simple, it simply is not for programming for a long time because it lacks a lot of important features, it's instead for quick edits.
There's absolutely no need for ed when you have sed and ex.
Con No persistent UI
You can only view portions of a file through search or regular expression commands. You cannot scan or scroll through a file using the available screen realestate.
It's not too dissimilar to only being about to use command-line tools such as grep and sed for editing a file. (in fact, both grep and sed are based on how interaction is performed with ed)