My first shell was csh. I put off switching to tcsh, but eventually did. After a while I grew used to the new features of tcsh, with tab completion being the most obvious. I then went back to csh on some machine and realized just how dependent on was on tab completion. It became frustrating to type all of the filename again, rather than just the first few characters<tab> follows perhaps by some another character+<tab> for disambiguation.

I don't want to go back to a shell without tab completion. Is tab completion extremely useful? Functionally, no. I can live without it. But I will avoid that if at all possible, and that's not something I felt before I used tab-completion.

I get the sense that IPython is the same way. It's not essential, but after many have used it they have the sense that they don't ever want to be without again.