From a technical point of view CAS2 SDRAM gives only about a 3% performance increase over CAS3 SDRAM, other things equal. That's like 20-30 MHz equivalent, which isn't very much, but which may be a very expensive 20-30 MHz if you have to buy a lot of premium grade RAM. OTOH, some of the premium grade RAM may run at CAS3 at much higher speeds than the generic CAS3 stuff, which may be why a lot of overclockers buy it.
Also, sometimes the manufacturers will label PC133 SDRAM as PC100 in order to meet market demand for the latter when yields of the former are very good.