In the speeder world we call a suicidal attack "noobing." It's a term coined for a player going after a kill with negative outcome odds. Nooby players may try this attempt to win believing anything above 30% is a worthwhile attempt because of the points in question. Players being noobed are always going to regress.
I myself am not a big fan of any negative probability sweep starterrs. For your first kill the worst odds worthwhile are upper 40% if you know for fact another player is going to win ... assuming he'll achieve 80% probability or more after your turn is definitely giving you odds to go for a 40% kill -- assuming you win 20% of the time if he loses, which is probably over estimating it unless you are playing with a bunch of sallies. A lot of players will even go after 60% probable sweeper kills and that would immediately equal your 40% chance... so probably you would eithre lower his chances to closer to 50 or try the 40% yourself (depending on a lot of things) if he has 70% and you are looking at a 40%..maybe it's time to try for it. Of course these are well thought out plays and not just some mindless suicide. I saw a guy win a 4v9 when I was about to take a freestyle game home. He knew it was his last hope and I don't blame him in the slightest. He went on to win it and I proceeded to throw things at my fridge, cuss out my dog, eat a bunch of ice cream and cry myself to sleep.
Other forms of suicide come in terminator games. When you see a high rank opponent players tend to suicide on them. I would know... I have seen it many times where they take a swing at me and the next turn or maybe 15 seconds later if its freestyle they are already out. It's really not a bad strategy, but I would recommend going for sweeper kills, which will be more profitable in the long run -- especially if you are winning the games and aren't getting knocked out on mistakes like noobing a high ranker.
Like kaskavel said, there is no reason to level games or suicide a player for anothre players benefit. I have had it happen to me multiple times just to end a 3 way stalemate game where they truce and hope to get lucky. There are also lots of instances where people will do these things in favor of a high ranker so they lose the minimum, giving him/her an easy clean up. This is a cowardly way to play. The only time you shold suicide is in a game where your troops/regions/bonuses and every chance have been exasperated especially in a 1v1 situation. It's not terrible to provide an easier clean up, but nobody is in the right saying you should give up in any position.