I find that Rex can fit a niche quite well in criminal decks as a cheap backup decoder, very similar to Faerie. It costs 3 more to install than Faerie but you will be able to use it at least once more than a copy of Faerie so I think that the install cost balances out. For example you might be using Yog.0 aided by Datasucker as your deck's main decoder, but you could put one or two copies of Rex in your deck to cover you for scenarios where you are low on Datasucker counters or your Yog.0 ends up being trashed by the corp. As others have said, the influence cost is way too high to use it in non-criminal decks.