Erm... Their moves are their choice, I'm not sure what your proposing, really.

I'm just making fun of myself in a sad, sad way.

Sometimes, there is a different agenda. For example, perhaps the reason someone is easy to stab is that they became obsessed with eliminating another player. I've seen that happen - once, I was the player being attacked...unmercifully...and without any semblance of sanity...*sob*

Each player has their own choice. I know situations where I've been almost certain my ally will stab, but ignored it. There's not always an option. Look at the other side: each player is in for their own good. If they were to confront an ex-ally, it might kill them, whilst hoping not to be stabbed keeps them to one enemy, and may give them time to prevent the attack.

They are just a victim of their friendly, optimistic personality. Keep em.

There's also the possibility that they can't prevent the stab. There's also the possibility that even if they try to prevent the stab they will still end up getting defeated. So if faced with

90% chance I get stabbed
If I get stabbed, I have a 25% chance of surviving
If I don't get stabbed and act aggressively towards my ally, he's going to stab me, giving me a 35% chance of surviving.

Or I take the 10% chance I don't get stabbed, and end up with a 2 way draw?

(All numbers are fictiticious, just an example of the possible thought

if people know a stab is coming and let it happen to attack someone else instead, then I would first check if there are any hints of multiaccounting.

if there are no such hints, then I would congratulate the stabber for being able to pull this off

The problem is that you get more and more likely to be stabbed the more and more hopeless putting up a fight is... :/

If you look at DbJ's numbers mathematically you get (This assumes this is the last chance for a successful stab).
If you don't pre-empt
Stabbed-survive 22.5%
Stabbed-die 67.5%
Not stabbed 10%

If you pre-empt the stab (This assume your ally has enough defence to not be weakened by aggressive moves if he did not stab simultaneously)
Pre-empt-survive 35%
Pre-empt-die 65%

Of course you then have to decide what the value of "survive" in the game is which depends on:
WTA v PPSC.
Chances of a stalemate against the stabber.
Size of the drawing alliance.

It is a very interesting question.

first, one would have to actually put numerical values on the likeliness of all move combinations that might come at you.

in the end, the "I will probably die" and "My chances are good" are the most accurate things that anyone can come up with

If you are going for the Ghost-Ratings, PPSC versus WTA is irrelavent, except that you could use the points in PPSC to try and get into bigger and better games.

They often do it because the despise someone else more.
In this game: http://phpdiplomacy.net/board.php?gameID=5137 I as a weak England kept a very strong Italy out of the Atlantic for ages, this allowed his smaller Austrian teammate to grow too big. The stab was obvious but Italy ignored it because he despised me and my "blocking" tricks.
After a lot of fighting, Italy even gave Austria the win. :-(
Although well played Austria, getting your victim to help you win is good diplomacy.

22.5% + 10% = 32.5% and that's very near the 35% chance you would survive if you pre-empt the suspected stab. I argue that there's more than a 10% chance of no stab coming your way in most cases so it's probably better not to pre-empt in most cases.

Maybe. interesting question

@FS - how do you *know* when someone is going to stab you? i would REALLY like to know that!!!! please share the secret with us. =D

if i know i'm about to get stabbed, i use my best diplomacy to try and counter it. :)

Draugnar, PPSC versus WTA is just as relevant in Ghost-rating as in points. Ghost-rating uses whichever system phpdiplomacy used.