That depends on what you're trying to achieve. At its most basic, Coreboot is a set of platform initialisation code. That corresponds to the PEI layer in an EFI image. Coreboot can then launch a bunch of different payloads, which include the ability to execute a kernel directly, boot a legacy BIOS (based on SeaBIOS) or even launch an EFI environment based on the Tiano Core release.
If all you want to do is boot Linux then Coreboot definitely lets you achieve that with less overhead, and if vendors used it as the basis for the PEI layer we'd benefit from having the source to their setup code and could use that to identify bugs. But we'd still run into the common BIOS issue that vendors *will* introduce bugs, and we still need to work around them. Even if we have the source code and can rebuild the full BIOS image with a bugfix, having someone flash a third-party BIOS image would probably void the manufacturer warranty. So we're still at the mercy of the vendors in terms of bugfixes, and the open source nature of Coreboot doesn't benefit us hugely in that respect.