It's not railroading, it is about balance. It goes with the saying "Jack of all trades, master of none." in that you can spread your effort and time into everything but you wont be great at much of anything. For example, how many people do you know are a doctor and in the evening go work at a garage changing breaks faster than anyone else?
True this is a game and we are making exceptions to the rule but the idea remains the same...you get so many points and feats to reflect your experience and can spread only so far... nothing prevents you from multiclassing but in the end you wont be as good at something as if you stayed one class.
A inquisitor is nice because you come close and if you know what you are doing you can break a barrier or two... That is where the fun is, my archer doesn't have pbm, snap shot..combat reflexes etc...but he doesn't really need it when he is hitting far harder.