some factions make it so that you must join them ( keepers, wild, phantoms, scra, rheinland military ) - but by default, you don t need to - in order to have fun. - i have been playing independent for a long time ( and still do, but less ) - and you can have lots of fun, but more and more game content becomes "locked" away by factions.
like when i started, it was no problem to buy practicly any ship there is and apply an appropriate RP to it. ( i had quite some ) now an appropriate RP isn t what you need for any ship anymore, but you must indeed join up or be shot on sight. ( and declared ooRP )
so, "yes" you must be in a faction to experience 100% of the game. without you ll have locked out content.