Some improvements have been made in the MBR code that ships with syslinux. One of them is, that the MBR code now starts with 0x33 0xC0 which seems to increase compatibility with some (braindead) BIOS implementations. See
The most recent change in the MBR code was in Dec. 2010. However, the syslinux MBR that ships with ms-sys is 10 years old (according to CVS).
If you are going to update, that begs the question what "ms-sys /dev/sda" should do when it encounters an older syslinux MBR, like it would be written by current ms-sys versions.