No, taking propanolol to rid a person of painful memories is not going to effect positive ones or other memories, because the person will only forget what they want to forget. It is unfair and judgmental for people who have not suffered trauma to decide that other people need to live with horrific memories of being raped or brutalized. It should be the victim's choice.
In theory, if a drug dampened a memory, it could affect long-term or short-term memory in the future. However, studies of propranolol indicate that the drug doesn't eliminate bad memories – it just seems to avert a typical startle response to phobic stimuli. Nothing indicates that there are long-lasting effects to memory, but the long-term effects of propranolol on memory have not yet been studied.