We aren't completely sure why the default value of the ServerTokens directive in Apache is Full rather than Minimal. It seems like Full is really useful only for debugging purposes. A probable reason for using ServerTokens Full is publicity: it means that Netcraft (http://netcraft.com/) and other similar survey services will count more Apache servers, which is good for all of us. In general, though, you really want to reveal as little information as possible to potential crackers.
Another approach is to modify the httpd sources to not reveal any unwanted information, so that all responses return an empty or phony Server: field.
Be aware, however, that there's no security by obscurity (as the old saying goes). Any determined cracker will eventually figure out what version of Apache is running and what third-party modules are built in.
You can see what information is revealed by your server by telneting to it and issuing some request. For example:
panic% telnet localhost 8080 Trying 127.0.0.1 Connected to localhost Escape character is '^]'. HEAD / HTTP/1.0 HTTP/1.1 200 OK Date: Sun, 16 Apr 2000 11:06:25 GMT Server: Apache/1.3.24 (Unix) mod_perl/1.26 mod_ssl/2.8.8 OpenSSL/0.9.6 [more lines snipped]
As you can see, a lot of information is revealed when ServerTokens Full has been specified.