I suspect your premise about gaining more points for winning than losing is slightly off. Generally speaking, once your point level stabilizes due to finding your actual skill level, your points will remain the same relative to others playing the game if you don't get any better/worse.
However, the total number of points everyone has will trend upward due to the Bonus Pool. This does mean it rewards playing more often, but only to a certain extent in a certain time period (a few games per week, I suspect). Once you've exhausted your bonus pool, your ranking points will again more-or-less stabilize based on your skill.
One of the reasons this was done was to encourage people to actually play the game. Since everyone's points are constantly going up, you have to play to keep up with your peers. If this artificial inflation of points due to the bonus pool wasn't in place, the player at the very top would - in theory - not have much reason to play; he's the best and his points will stay the highest until someone usurps him. This way, he has to keep playing to maintain his status. As with any fair matchmaking system, the more people playing, the better, and I think Blizzard is trying to encourage more people to play.
However, none of this is cut and dry so there's no one reason for any of these decisions; likewise it's not at all obvious whether this system is superior to the traditional ranking system you mention.
That said, here are some good articles on the subject:
Note also that the points you see displayed and used for your ranking may be separate from the underlying statistic used for matching you up against other players. The two stats should converge on relatively the same thing, but the Bonus Pool adds the extra incentive to play more often.
At the time of this writing there are:
Type Total Russia Taiwan SE Asia Europe Korea NA Latin America
All 25,670 862 1,707 1,299 7,371 2,678 11,380 373
Broken down by Type and League (approximate numbers)
X 1v1 2v2 3v3 4v4 2Rand 3Rand 4Rand
Diamond 700 200 50 25 200 200 100
Plat 1400 700 200 50 400 400 300
Gold 1900 1100 400 200 700 700 500
Silver 1900 1300 500 200 800 700 500
Bronze 3500 2600 1600 600 1400 800 400
Total 9400 5900 2700 1050 3500 2800 1800
This data was tabulated by myself based on the website: http://sc2ranks.com/
As to what that split represents, I refer you to this question:
What percentage of players get into each league tier
Best Answer
According to Blizzard Forums:
By not playing for an entire season, you wiped your MMR clean.
Therefore it will take sometime to get to upper leagues. Just keep playing, you eventually will end up where you belong according to your MMR.
For answering your question:
So, you're safe playing one day a week. Your MMR won't decay.