"if modified since" and google

I've been practicing a bit of php lately, especially things relating to http headers. While I was at first a bit annoyed that there was no easy way to detect the http headers in php4. there's a get_headers, but apparently this is only implemented in php5.

I did, however, stumble into something else while searching for it. since I was going over and over the headers section in the php manual, I noticed that you can send out the last modified header.

since this could be so easily abused (you'd just have to have it send out the correctly modified current date/time), I assumed there was no way google or other search engines could possibly use this much at all. however, apparently I'm wrong, googleguy endorses its use.

interesting, to say the least. drawbacks of doing a modified since "hack"? more bandwidth (if you want some sort of medium, you could have it be changed on a certain frequency, not all the time). but hey, that's not too much to ask for in exchange for more frequent visits, is it?