Priest is one of the first class other than warriors that is available. It makes sense for new players to think that picking that class will still allow them to play the game at its fullest. Of course you don't gain military prestige which would allow you to command more troops, which makes sense, but the prestige restrictions for titles were always meant to prevent extremely new characters to be propelled in command positions due to family name recognition or, worse, takeover by multis.
If it was actually the intention to prevent priests from becoming Lords, the text should be much clearer when you switch class. Something along the line of "While other nobles will admit your inclusion in conversation and show some respect to you due to respect for the divine, unless you have shown military prowess in the past you will be treated as no more than a commoner, with no action you take being worthy of increasing your honour or prestige in their eyes. By taking this path, you accept that you will never rise above the station you have achieved already."
Although I don't think that was the intention.
