People should donate to cancer charities if they have the money to do so. Cancer does not get enough funding from the government and needs additional help with funding. Cancer will probably affect either yourself, or someone that you know closely and you should be allowed to donate so people do not have to suffer through that.
If a person can financial afford to donate to cancer charities and it is a cause that they care about, then they should donate to cancer charities. The medical field is still rife with people eager to learn more about cancer, which could eventually lead to a cure, but these professionals need funding to complete their work.