And what you expect to find there? It's not a FO1&2, you can't just run to Sent Louis and grab sniper rifle (most likely you WILL be killed, if you run into there BEFORE you really given that mission). So yeah, no shit-short-cut technique
Also, some missions (at least 1/3 of them) have exit grid NOT in the same place, in which you appear... What will you do, if you appear at Macomb start position without finishing ever Freeport? It will be impossible to reach exit grid here (even if we don't count "deliver the hummer" condition).