After a bit of chat on the IRC, I decided to give my two cents.
This proposal can be viewed two ways. First, there's the question: Is there a need for this from a roleplaying perspective? Secondly, there's the question: Is there a need for this from a mechanical perspective?
Let's start with the mechanical perspective. As Smite Good and a few cleric spells only work on enemies of the opposite alignment, having an area with creatures that have Good alignment would be helpful to ease the PVE off Evil aligned clergy and blackguards.
However, the question still stands: Is there a roleplaying reason for this? I say no, and I reason it with the definitions and expected behaviours of alignments.
First off, evil is not a faction. There is no "Team evil versus team good", as evil can be the most varied form of behaviour after Neutral. To gain an evil alignment, all you have to do is actively act in a way that, knowingly, causes others undue discomfort, financial difficulties or death. To be evil you do not need to fight "team good", you just need to play as the average adventurer with even less mercy and even more greed.
Now, you may argue that there are churches and factions that are evil aligned. Yes. However, more often than not (taking Drow, Banite, Bhaalite, Orc, Talassan churches as my examples), these sects often tell their members to fight amongst themselves, to prove themselves the most worthy of their god. Thus, Evil killing Evil is often justified. There was a DM event (amongst many!) that was of an evil tint where two Banites stood at odds, trying to see if they need to kill the other or just simply subjugate/divert them to their own goals.
So in essence, as evil you do whatever you might please. Lawful alignment might limit you, but not too much.
Let's take grinding as an example. Xvarts. Xvarts have a village which you choose to visit and slaughter every member of. Xvarts are evil. However, Good would never murder indiscriminately. At most, that'd be Neutral if not outright evil.
"Let's go kill goblins. I hate them." - Evil action (act of killing) with evil justification: Net evil.
"Let's go kill goblins. We need to make sure they won't hurt anyone else." - Evil action (act of killing) with Good justification: Net neutral, leaning evil. (Yes! Leaning evil)
"Let's do something about those goblins, scare them off or something. We need to make sure they won't hurt anyone else." Good aligned approach, possibly resulting in Evil action (act of killing), Good aligned justification: Net good.
The above shows that most acts of grinding as it is possible without a DM present will either result in Neutral, if not outright Evil alignment shifts as according to
http://easydamus.com/. As such, there is no need for a "Good" area as you already get the necessary evil points to keep yourself from falling back to Neutral (due to Apathy to your alignment).
As a side tangent, to the evil who claim there's too many good aligned characters and it's unfair to them, I would, based on
http://easydamus.com/, argue that many of the good aligned PCs, -even paladins, dwarves and elves- should in fact be Neutral aligned, not Good, if not outright evil. Good Alignment is a Luxury to have that requires active pursuit of Alturism, actively acting with a respect for all life (Shevarashites!) and personal sacrifice to go against the currents that is society to uphold the previous (a Good aligned elf for example will stand up to his/her peers when they wish to execute an unarmed, obviously non-threatening drow even if it means to lose their respect or his/her standing in society. To not do so would be Apathy towards Alignment, and incur a detoriation towards neutral. And yes, I've seen this happen.). Emphasis on going against the currents that is society to uphold the previous, as society, due to fear, ignorance and leadership is True Neutral at best.
