On Windows 7 when the defrag service is enabled, it doesn't take SSDs into account.
It is possible to see this in scheduled defrag settings. SSDs can't be selected at all, as they do not even appear in the list.
So in case of a system with both HDD and SDD installed (I guess the great majority of desktop systems), it's kind of stupid to disable this service completely as in this way hard disks won't be defragmented as needed.
On Windows 8 the OS performs either scheduled TRIM or defragmentation depending on the drive (SDD/HDD). So again, its related service should not be disabled.
There might be rare cases where SSDs aren't properly detected as such, like older/cheap SSD or RAID controllers, so if this is your case it might be worth to check settings to make sure they are suitable for your hardware. With relatively modern hardware this shouldn't be necessary though. By the way, I have an X25-E SSD in one of my systems and on Windows 7 it does get detected as an SSD, so it's not that cutting-edge hardware is a requirement for that.