It might be that FICA was really meant to be paid by the employers but they figured out a way to dump it on employees. Nonetheless, from an accounting and economic standpoint FICA is very important to keep the SSA solvent (afaik about $578B in 2007). AFIK the SSA is highly solvent. If you get that the person named on the SS card is a legal entity and if you get what SSA is really in the business of doing (yeah insurance isnt the right word because insurance is technically commercial--SSA provides the something like insurance but technically non-commercially).
Its worth noting that from what I've gathered, the SSA invests all of its money in U.S. treasury securities.