I suppose that size and speed is an issue for a lot of folks looking for a client-side filter. My machine has cycles to burn while I'm not home and spam keeps tricking in, so it will be all finished analysing when I get around to checking the in-box. So for someone who auto-checks the POP all day, a more server-like solution is feasable. 1GB of disk space is nothing... but weighting down my machine with a background task while I'm doing something interactivly is a bigger deal.