Its not required, but having a CFI will help in the interview and is a good way to build time if you need it to get to the ATP mins. Also helps in training as well, particularly the CFII. The Indoc test at my airline is a lot of AIM stuff that is easy if you have a CFII background because you teach it so much. Also a good thing to have a current CFI in case of furlough or if you have trouble finding a job. Lots of opportunities for CFIs right now.