Every bit of information leaked out about your server could potentially be problematic. Even the apache banner the leaks out if you dont disable it in the httpd.conf could lead to a compromise. Go to exploit-db.com and look at, for example, all the vulnerabilities against Apache. Note that most affect specific versions. So now with phpinfo exposing that info its simple for a script kiddie to automatically attack your system. Which even if its patched can still cause problems, at least traffic disruptions. Whereas if they dont have a version then they won't generally run anything. Many apache mods have had issues in the past - another thing phpinfo exposes. Certain local exploits run on certain kernels - phpinfo again...
It also shows what db you are using - so immediately an attacker knows which commands to try to use to conduct sql injection attacks. And certain versions are vulnerable to specific exploits.
You also can guess from phpinfo such info as user names, the format of user names, the directory structure...some attacks you need to know the directory structure, like PUT attacks, where usually the attacker has to make an educated guess, but phpinfo puts that info out there.
Even such info as the server administrator who you can then google and maybe find their security issues on StackExchange...
Basically, phpinfo is a big fat menu for any potentional attacker out there. Cover that **** up. And while you are at it, hide your banner versions unless you need that info for some reason.