If you have physical access to the machine, it doesn't matter. You can rewrite the BIOS. And then, yes, it is an advantage to malware authors if there's only a couple of kinds of BIOS, because their malware only has to support those kinds. So yes, reuse of code becomes a "problem" for the rest of us if viewed from that perspective. It's not clear though that life would be any better for users overall if there were more kinds of BIOS. As bad as Phoenix, Award et al can be at making BIOS that works, I shudder when I imagine vendors rolling their own. I'll live with the disease, thanks.
Yeah, I agree with with regards to the physical access vector. I have a background doing IT in a DOD TS/SCI environment for three years and a TS environment for eight with DOE. Our (those of use who knew what we were doing) had the philosophy that if you had physical access to a system then you could pwn it. AT DOE it wasn't our duty to design systems with any consideration of the "insider threat" unless it was for the use of FORNATs. Systems for US use relied mostly upon personnel and site physical security.
I do disagree that a greater number of targets being more burdensome for the black hats outweighs the security benefits of supporting a smaller code base. The former is merely supposed security through obscurity. A basic rule of thumb of security is to minimize the attack surface. One of the primary strategies to accomplish this with regard to information security in a software environment is to reduce the amount of code running.