technically you don't NEED a top FI ... if you already have established guys at ML SS/CF/3B/2B and if you haven't recently drafted / IFA-signed any SS's or CF's then there's really no reason to spend $5M on a guy you're not going to actually use. If you have recently drafted a top SS etc then a top FI is imperative to their minor league development so that they do actually qualify at SS in the bigs as opposed to RF or whatever
Also there's an oddity in situations where you acquire a player from a team with a FI ~50 IQ, or if you inherit a franchise that has had FI ~50 for 1+ seasons, and you put those players into an environment with FI 90+ then the players will regain normal development regardless of how their past trended. You can have players who were completely stagnant for 2+ seasons begin improving again every cycle. Thus it can be profitable to target some those teams for trade, because there is some "hidden" value in their under-developed fielders