OK, so lets clarify this. A player who shows up in the FSO arena with no prior contact will a place be found for them or will they be turned away? AKKuya seems to be describing a situation where they will not be turned away. If true, despite the inconvenience, this is good. A player new to the whole concept would get to sample it and a player like me who doesn't know till the last hour whether or not they can participate will show up if they can. If this is not the case, no problem, but it really needs to be clarified I think.
By definition Walk-ons are not allowed in FSO. FSO and Scenario are pretty organized and many expect other pilots to know what is going on. Squads and pilots are held to a pretty high standard in this regard.
In days of old if you showed up on game-night in FSO you could not ask in Country/All/200 to find a squad, it had to be pre-arranged. This is still the case but we are much more lenient on it. Squads are free to take pilots in but that squad is responsible for their behavior and getting them suited up and in the correct aircraft. If you are not in uniform by launch time, you are grounded for the event.
Scenarios also require pilots to be in squads for log purposes, however it is somewhat less of an issue as flight is always open for the entire frame and people come and go. Hence why walk-ons are allowed.
FLS is right - best practice is to get it pre-arranged as early as possible. This is good for both the squad and pilot as it is less hectic close to launch time.
I may think about re-wording the rule on the FSO Rules page to better clarify for the times.