Why is there a minimum rating gain when you win? Fide has a rule saying that if one player is rated more than 400 points higher than his opponent, their difference should be set to 400 when calculating rating gain/loss. That means no matter how weak the opposition is, a GM (or anyone) will receive minimum 0.8 rating points for winning. What is the reason behind this?

To me it just seems like a giant weakness in the rating system, making any strong GM able to get as high rating as he wants just by playing weak opponents. Wouldn't it be better if the gain approached 0 when the difference increased (like the basic formula for calculating rating actually does)?

EDIT: As most of the answers don't really seem to get the point, I will add an example which points out the problem:

One of the top 10 players in the world (say Magnus Carlsen), wants to increase his rating to 3000. He has a good friend who has 1200 in rating, and who is willing to play a match of 200 games (2 each day for 100 days) against him. His friend will of course play his best, so this is by no means considered cheating.

According to https://wismuth.com/elo/calculator.html, Magnus will have way more than 99 % chance of winning a single game, but for simplicity, let's say he has 99 % chance of winning a game (which is an understatement). If we then forget about draws (for simplicity), Magnus would be expected a score of 198/200. That means a rating gain of 0.8*198 - 9.2*2 = 140 rating points. That is if he scores as expected. And he used less than 1/3 of a year, and is extremely close to 3000 already. What if he does it again? (And again?) Even if he plays really poorly and loses 5 and draws another 5 out of the 200 games, he will gain 85 rating points. That is by scoring FAR below expected.

If there was no rating difference cap, playing an expected result (say 198/200), would by definition give a rating gain/loss of 0. So who came up with the idea of making a loop-hole for any top GM to become extremely high rated?

Of course it would be looked at as bad sportmanship to do something like this, and I doubt any of the top players of the world would consider doing so, but that doesn't justify the possibility being there. In a serious sport with a serious rating system there shouldn't be these kind of loop-holes.

Can anyone find any arguments justifying this rule?
Fide has a rule saying that if one player is rated more than 400
  points higher than his opponent, their difference should be set to 400
  when calculating rating gain/loss. That means no matter how weak the
  opposition is, a GM (or anyone) will receive minimum 0.8 rating points
  for winning. What is the reason behind this?


This is an historical anomaly. Currently, apart from your first ever rating, every game is rated individually. This means that you can never lose rating points by winning a game (LRPBWAG). It wasn't always so.

I don't remember exactly when the change happened, 2008 I think, but before this ratings were calculated per tournament. What happened was that your rating was calculated using your score for the tournament, the average of your opponents' ratings and your expected score against them. This opened up the possibility of winning the tournament but losing rating points because some low rated players in the tournament, who you crushed, lowered the average rating for the tournament so much that even though you scored well against similarly rated players as yourself your rating went down.

The original solution to this was to limit the difference to 350 points. Later this was changed to 400. Now that games are rated individually it could be scrapped.

There was an article on Chessbase 6 years ago discussing this point between Jeff Sonas, Ken Thompson and John Nunn.

Part of John Nunn's contribution is worth quoting -


  The original rating rules had no 350-point rule. It must be remembered
  that in 1970 the rating list was very small and it was never envisaged
  that it would be extended substantially. Only top tournaments were
  rated (and these were invariably all-play-all events) and all
  calculations were done by hand. The 'Losing Rating Points For Winning
  A Game' problem didn't really arise. The top players always skipped
  the first round or two of an Olympiad, and in any case any weak
  opponents would be unrated, so the games simply didn't count.
  
  But times changed, the rating list expanded and started including more
  and more weaker players. Open tournaments appeared, and the LRPFWAG
  problem started to become significant. I don't remember exactly when
  the 350-point rule was introduced to deal with this, but I believe it
  was around 1980. Later, in 1988 or 1989, Karpov was unhappy about
  losing points when he had won a tournament, arguing that the aim of
  taking part in a tournament was to win it, and if you have achieved
  that then you shouldn't lose points through not having won by a large
  margin. For a time the 'Karpov rule' was in effect, that you couldn't
  lose points for winning an event. This was later scrapped.
  
  Rating games individually is a fairly recent innovation, and for the
  vast bulk of the lifetime of the Elo system it was done on a
  tournament basis (because it made the calculations easier, especially
  for all-play-all events). The current rule is in a way a hangover from
  the earlier times, but I think there is some logic in it, although one
  can argue over the exact details. Rapidly improving young players, for
  example, can be massively underrated.