Perhaps Mavhc's taking the mick a bit, but just in case:
"They use less power."
In the same way that a toy glider uses less power than a fighter jet. It's just that you sometimes only need a glider, other times you need a jet or somewhere in between.
"They're easier to program in machine code."
It's just a matter of taste. X86 isn't so bad, you just spend 20 minutes accepting it and getting over it. The segmentation is a headache, but it's not a show stopper.
"They're not evolved from 25 year old designs and therefore not (as) full of hacks."
In your opinion. Unless you know the ins and outs of the recent Pentium designs.
"They're simpler and therefore more understandable."
This takes us back to the glider vs. jet fighter argument.
"They don't have a BIOS"
What the hell is that supposed to mean? A BIOS is typically a simple, ROM based program that initialises a machine and then finds the start of the operating system, or a bootloader for the OS. ARM based systems have them (for example, my StrongARM Netwinder) just as much as any other system. For RISC OS, the 'BIOS' and the OS are rolled into one because it's all supplied in ROM. Someone else mentioned CMOS, which unfortunately also makes no sense. CMOS refers to a type of memory that can be kept persistent by a battery back up. It's not exclusive to a BIOS.
As it happens, Intel are announcing some new 'low power' cores, which I'm about to write up. In short (and in my opinion), ARM is best suited for embedded and 'fatter-than-ARM' (for example, Intel) is best suited for non-embedded. If you want to use RISC OS optimally on a non-embedded platform, you need a processor that's not aimed at the embedded world. Feel free to correct me, this is just my take on things.