There was this comment on some blog that suggested that given that now we're in 2007 and most operating systems that matter support async i/o. There shouldn't be a need for multiple db writers.
Though I agree in principal with that. However, I have observed in my testing using an OLTP workload on a Sun V40z RHEL4 64bit Performance on Oracle that it did help performance when using multiple dbwriters as many as cores on the system. I thought I'd pass this info on for any one that may be wondering.