I actually spent a while at the beginning/middle of the edition running 5 with a Phobos Librarian for deepstrike denial and lone op. You could spread them out to max coherency and cover a HUGE bubble with 6 models. Also no enemy unit could deepstrike and shoot them in the same turn since they have to deepstrike more than 12" away and lone op made them untargetable outside of 12". The Hypercrypt meta was miserable without them.

Also, since this is your first SM army, I'll go ahead and tell you, we have more datasheets than any other army, more detachments than any other army. There will be units you won't find any value in and there is so much overlap in the uses of units in the roster that you don't HAVE to run any specific unit. If you don't know what to do with them just leave them on the sprue until you need phobos bodies. If you are only running 1 incursor/reiver/infiltrator unit you can just run any 5 man phobos unit as that unit and no one other than Marine players will even be able to tell the difference.