Primarily because client side software that serves a similar function is bulky, harder to configure, and almost never free. If your browser becomes compromised

Primarily because client side software that serves a similar function is bulky, harder to configure, and almost never free. If your browser becomes compromised there's a fine chance its built-in phishing/malware filter will be broken or disabled. It's unlikely a browser exploit could change your DNS settings -- if it could you've got much bigger problems. From a performance point of view many ISP name servers are either oversubscribed or simply not fine tuned. There are many tricks to DNS configuration on busy servers and OpenDNS does a good job keeping their servers fast and responsive. Additionally your ISP may not be giving you the best possible DNS servers. Many ISPs are getting involved in the ISP sponsored custom redirect game. While OpenDNS does indulge in this to a degree their service doesn't seem to take a hit from it which is something I cannot say about other custom redirects.