I agree servers are attractive targets. But I think the main reason they're attractive is because they have a lot of potentially high-value data on them, depending on what they are. A server, by itself, is a valuable target. Clients, and in particular clients for home users, are really just valuable en masse. Virus, worm and/or trojan-style malware makes a lot of sense for client machines, where an attacker probably isn't going to go to any great trouble to take over any particular machine- they just want a lot of machines. And once they get control of a client machine, they're not going to spend hours figuring out what data on it is valuable. Attacks on clients just tend to be a lot more automated from top to bottom.
Because servers are valuable individually, an attacker will spend more time on it. Mainly, the method of attack will be different. They'll craft their own exploit code to get in. They probably won't just add it to a botnet they control. And, I think you have to expect that any time an attacker does something "noisy" on a server, like using it to send out lots of spam, it will get detected rapidly. But, I'm sure there are situations where that happens. I sure there are lots of insecure SMTP servers out there that get hacked into each day.