The Milwaukee win isn't actually the best case of a reason to bring in this ruling, also known as the "Slaughter Rule." It was "only" 10-0 after six innings, before the Brewers piled on ten more. (A better example would be the Texas Rangers 30-3 win against the Baltimore Orioles during the 2007 season.)
Regardless, it is still something that Bud Selig may wish to investigate further. As such, here are the arguments for and against bringing the mercy rule in:
There is no game clock in baseball:
Unlike Football, Hockey and Basketball, there is no time-limit on how long a game lasts, meaning, in theory, a team could continue to pile on the humiliation with no referee to jump in boxing style and stop the contest.
It could be argued that it's very rare that an athlete wants to humiliate a fellow professional. Surely you can't feel particularly good about beating up on another team, especially if you have friends playing for the opposition.
Run differential doesn't matter at the end of the season:
Whether a team wins 1-0 or 10-0, it has no bearing on if they qualify for the playoffs at the end of the regular season. If two teams are tied for the division lead (assuming both aren't assured of a playoff spot) then they contest a one-game playoff.
The New York Yankees have won 27 World Series:
Okay, that last one was a feeble attempt at humor, but there has to be some way of stopping the evil empire piling up the championships.
Games are rarely that one-sided:
A 20-0 or a 30-3 happens so rarely, is it really necessary to employ a rule for that odd occasion when a team is getting annihilated? Just sit back and enjoy the hitting clinic (unless you support the losing side).
The rules are the rules:
Everyone knows what the rules are heading into a game. Why should they be changed simply to benefit a team who is getting embarrassed? The team has nine players, just like the other side. Maybe the losing side should take the radical approach of actually getting the others team's players out.
Players would stop trying:
Further to the above, a mercy rule might make players think that it's okay to just give up when things aren't going their way. In sports, as with most jobs, you should always put in the effort and try your hardest to turn things around, even if you appear to be in a hopeless situation. Plus, some of the best lessons in life are learned in adversity.
It would mean less opportunities for reserves to play:
One of the first things you find when a losing team is being well-beaten, is that they bring in the bench players and give them an opportunity to play. If the mercy rule was brought in, it would mean less chances for the reserves to get valuable experience and prove their worth.
As you can see, there are several arguments for and against employing a mercy rule in professional baseball. Personally, I don't think one is necessary. However, come back and ask me again if the Mariners ever lose 25-0 to the Yankees. (And the chances are I would also throw in the need for a Salary Cap for good measure.)