Yes, security is a continuum, and there is no such thing as perfect security. There are many security people (or people working on security technologies) who often forget that, typically because they overly focused on some specific threat and ignore the bigger picture (or, it is not their job to consider the bigger picture), or alternatively because they will not bear the costs of what they are proposing/implementing.
There are many security steps you might take which, while not offering perfect security, are still worthwhile. E.g. it is worth locking my door, even though this would not stop someone who is prepared to bash the door in. However, fitting a lock to my front door would *not* be the best prioritisation of my resources if the windows and other external entrances still hadn't had window frames & glass, & doors fitted.
I.e. security measures have to be seen in context. Any given security measure can not be said to be worthwhile of itself. It must be assessed in context with the threat - realistic threats - AND in context with the rest of the system the security measure will be put in place with.
General purpose OSes, and the software they run today are ridiculously insecure. Secure Boot can NOT secure a Linux system from at-boot infection from the class of attackers who can write rootkits or firmware exploits. Because it is equally possible to write an exploit for early, privileged userspace, and use a general runtime exploit to install it. "Secure Boot" buys you *nothing* in this context. Similarly, I don't believe "Secure Boot" will do anything to make Windows 8 more resistant to viruses and malware.
However, "Secure Boot" CAN prevent the average *OWNER* of the machine from using the machine - once the "restricted boot" bit is set by some other party (e.g. the hardware vendor). In this context, the "Secure Boot" technology does indeed generally work. It makes "Restricted Boot" work.
"Secure Boot" will not help security on general purpose Linux machines, from capable attackers. The main class of people who could possibly be stymied by "Secure Boot" are the users. All the other instances of this technology, from Tivo to game-consoles, to phones, are being used to prevent *OWNERS* from having full access to their hardware.
To think "Secure Boot" is about helping owners secure their own machines against others seems very very naïve to me.