The si is an artifact that replicates. Since they are artifacts, they are:
1) indestructible - wield them as a melee weapon to prevent corrosion/damage from
jellies if nothing better is available
2) valuable - sell them to shopkeepers: potentially limitless cash
3) sacrificial fodder - sacrifice them to the gods, although it may be more beneficial to sell them and sacrifice the gold
All of these properties together make sis rather nice items. Always keep at least one.
Note that sis do not count as an artifact as far as the demented ratling is concerned. Also never forget that they replicate. They can get heavy after a while. A particularly nasty effect results if sis are brought into the ID. On a long trip through the ID, to find MaLaKaI, or Filk, if it has worked out that way, sis will replicate to the point where they become annoyingly, or impossibly, heavy. This would not be a problem if the PC could simply drop them. Unfortunately, they jump back into the PC's backpack before he leaves a level, no matter where on the level they were dropped. Furthermore, monsters in the ID will not pick up sis, so they cannot be gotten rid of that way.
Think very carefully before bringing sis into the ID on an anticipated long trip. Many castings of Strength of Atlas is virtually the only guaranteed safe method of dealing with them. Girdles of carrying and girdles of greed, for example, can be cursed by a hag or destroyed outright.