Why is there a length limit on edits? One con to the limit is that it's not possible to correct some simple typos e.g. 'ture' for 'true'. Are there pros that outweigh this?
This is the limit on edits:
Users with 2K+ reputation: These users have the edit privilege, implying they can make edits without it requiring approval by peers;
Users with < 2K reputation: These users are subject to peer-reviewed edits (by those above) that should be substantive. Substantive here means at "at least 6 characters".
First of all, the motivation behind this is put into perspective by the SE founder Jeff Atwood:
As a < 2k rep user, you should make reasonably substantive edits. The approval cost for your edit is not free, as it costs the attention of one or more users who have to look at your edit and think about it. This cost is high for extremely trivial edits.
This will not be changing.
If you want to make single-character trivial edits, earn 2k rep.
We are a small community relative to SO and friends, and therefore mentioning "it costs the attention of one or more users who have to looad at your edit and think about it. This cost is high" seems like a non-issue.
On the downside, some users who experience the gain in reputation from suggesting an approved edit (+2) sometimes do go crazy and suggest like it's nobody's business. Yes, incentives drive people to do this. :)
I agree with your mention of the con to edit-limits, because the downside of a lot of small problems (like typos) leaves you with exactly that - a bunch of posts with small problems in them. There doesn't seem to be much upside to this edit-limit from your perspective (having < 2K reputation), but perhaps that's just it: earn the privilege through reputation...
Other options and/or opinions have been put forth and are worth the read: