I think there's both pros and cons here, obviously it can be nice to be able to clean up any mistakes or misclicks. But it also means any criminal can delete evidence posted on twitter, any deals being set up can later be deleted etc. In my opinion twitter is balanced because it allows everyone to connect with eachother without needing to know anything about the person they're connect to, but also leaves a trail. Adding a delete function would remove this trail and could basically make twitter text messages without numbers, where you can have a quick convo, then delete it all.
Imo that trail is more important than a delete function.