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, sis will replicate to the point where they become annoyingly heavy. This would not be a problem if the PC could simply drop most of them. Unfortunately, they jump back into the PC's backpack before he leaves a level, no matter where on the level they were dropped. Bottom line: *never* bring sis into the ID.