You can install and use rEFIt on an Intel Mac. Of course it's a good idea to keep a bootable OS X partition if no other reason than to update firmware.
I've been running Ubuntu as my only OS on my MacBook for over 18 months now, without an OS X install and without rEFIt. Neither is essential for using a MacBook, and anybody who says anything to the contrary is clueless.
That said, I did boot my machine from the OS X installer DVD a couple of times to bless the Ubuntu partition into the EFI after reinstalling the system (following a hard disk upgrade and/or as a spring-cleaning measure). This wasn't strictly necessary, it just cut about 20 seconds from the boot time. With Grub2 having finally found its way into Karmic, even that won't be necessary any more.
As for firmware updates, these may require a running OS X install (which can be on an external drive). For my machine, however, there haven't been any updates that would have been relevant for Linux and there probably never will be.
To tell the truth I don't know why anyone would pay for a Mac and not use OS X.
Well, people are different, you know. I like my MacBook's look and feel, the slick and intelligent design, those little things like all the slots and connectors being on one side, the power adapter with its MagSafe mechanism (I tend to stumble over cables and to go berserk with vacuum cleaners), the touchpad with all its great little secrets ... Back when I bought my MacBook it didn't cost much more than a comparable high-end Windows notebook, like a Sony Vaio, with the added benefit that it's not a Sony (bah) and didn't come with a Vista tax (double-bah).
On the other hand, I don't have much love nor any need for OS X. I tried it, I didn't like it, it's somehow not compatible with the brain-half that's in command up in my head. Linux, while certainly not being for everybody, is just the right thing for me, the freedom, the power, even including the seemingly endless fight against numerous shortcomings great and small. I need that, all of it, I couldn't live without it. I don't want a computer that "just works", I want a computer that works exactly the way I want it to work, whatever it takes. And I've never been closer to that ideal than with Ubuntu on my MacBook. Which, by the way, works extremely well by now.