I only know of three partition schemes: MBR, GPT, and APM. (These are the ones Apple's Disk Utility can create.)
MBR (Master Boot Record) is used historically by Windows (and, therefore, by most computer manufacturers). I'm not certain if you can boot a Mac from an MBR disk; I don't think so.
GPT (GUID Partition Table) is generally used in conjunction with Intel's EFI BIOS replacement. As such, it is mostly used by Apple currently in their Intel Macs. Any Mac running 10.4 or above can read a GPT disk; only Intel Macs can boot from them. If a computer has EFI, you can boot Windows on a GPT disk.
APM (Apple Partition Map) is what Apple used before the Intel switch. It's been used since 1989 in the Macintosh SE. Intel Macs can boot from and read APM disks.
I'd say the assertion you made in the question ("GPT is more useful than MBR") is incorrect: you can certainly boot more computers with an MBR disk than one using GPT. GPT is the way of the future, but it may take awhile for the major PC manufacturers to switch to EFI from BIOS.
Your Time Machine disk is almost certainly uses GPT, but that's totally irrelevant as you'll never boot from it.