They can be a great vehicle for character expression. A disadvantage seems somehow less restrictive when brought about by a character's own convictions. It is also true that SIDs are, by definition, easier to reasonably be rid of. A good way to develop a character is to allow him to change in major ways. Codes of Honor, in particular, can change in an interesting way, as the character becomes bitter about his former virtue, or finds another way of living.
Pride is a factor, too.
Some players find it easier to stand behind a character's actions when he is motivated by honor or a bond of fellowship. Developing a PC's self-imposed limits early on is important, and will often fill that empty spot in the disadvantage list. Just a little thought will often reveal one or two aspects of a character that can be hammered into a perfectly legitimate personal Code of Honor. The major problem of SIDs is also their strength. These are individual traits, best when custom-tailored to the player's creation.