What are the disadvantages and advantages to terminal cancer patients using morphine to manage pain?

Many. Every patient responds to different opioids differently. Opioids also treat different types of pain with various levels of success, so would check with his/her doctor to see how well might work for the type of pain being treated (bone pain, visceral pain, neuropathic pain, etc.). Common pros are excellent pain relief for common pain syndromes; cons: constipation, temporary nausea, and more.
Serious illness/pain. You will have a better quality of life, and be able to visit with loved ones and be more active. It is generally believed that if your heart is not pounding and you are not gasping with pain, that you will live longer as well.