In that situation, as a consumer, I'm actually pretty jazzed about living inside Apple's walled garden on my iPhone. The phone refusing to run unsigned code gives me a little bit more security.
It also largely defeats the scenario where I can hire someone to modify my phone's software. But I'm OK with that tradeoff, because I think it puts me on the side of greatest benefit to myself. I suspect most non-programmer users would fall in the same camp.
...And it's free software with no arbitrary usage restrictions. Meaning you can actually audit it if you so desire, and in fact you'll be doing a great service to users and developers in the process.
It's a rational choice given the complexity, but makes you totally vulnerable to your feu lord. And their feu lord.
By the way, there is no necessary conflict between signing and software freedom if the system allows you to replace the master keys. Matthew Garrett is doing a collection of work to enable Linux users to use UEFI and TPM hardware to secure themselves against malware.
It does not prevent flaws from being discovered. It slows down the discovery of some flaws, but this also means that the people who discover them can exploit them for years if they're malicious, because flaws are equally hard to discover as part of a well-intentioned an audit as they are as part of an evil plan.
It also allows flaws that have been intentionally introduced to be hidden from sight for a long time -- and allows companies who do so to escape untarnished; since the commit logs are not available (and the source code is often only available in disassembled form), they can always claim it's a bug, not a feature.
It does not provide additional safety, it provides only an illusion of safety -- while making actual safety a lot harder to implement, for everyone.
You could argue that Firefox leaves open vulnerabilities to things like Superfish (that Windows root CA didn't actually affect Firefox in most cases since Firefox keeps its own list), but you can take the signing concept as low-level as you like, including requiring a signed OS with a hard-coded, single software delivery mechanism which could still be fully GPL.
You trade your freedom against short-term convenience. On the long-term it means that you have to accept all decisions Apple takes, even if you dislike them.