Leigh Jasper (Co-founder and CEO of Aconex) has written an interesting article discussing the perils of deleting records from collaboration based systems. Leigh talks about the integrity of the system. If you cannot be sure that a document in a control system will still exist at some point in the future, then that is a risk legally but also for business value.
I believe the same is true from a spatial and communication point of view. For example, if a user can delete a 'well' entity from a gas field, and it just vanishes from the screen, then the total solution can be questioned. There needs to be an audit control, so if an object is 'retired', users can still see it and its associated information. They can find out why it was 'retired', by who and when. You also don't want items being deleted unintentionally by error and never seen again.
The same is true about elements in a communication thread. If users can remove elements of the thread then the integrity will be questioned. Ultimately a business decision is made based upon all the inputs, but if one or more inputs are deleted, then the decision may not be the best one for the business. Post-decision deletions can also make future diagnosis problematic – for example tracking back in time to ascertain why a decision was made becomes impossible if users can delete information entered previously.