I don't use the conversation view myself but just got a question from an annoyed customer who "doesn't want to have those weird struck through mails in his mailbox". He meant the deleted mails which are displayed in the thread in conversation view. And now that I look at it I understand that it can be annoying.
I couldn't find an option to disable this behaviour but sometimes those options are hidden very well. Before I file an RFE on Bugzilla for an already existing feature, does somebody know if this exists already?