I'm not certain that it is so much "multi-processor" design, so much as modern hardware designed
You're getting real hardcore with this kind of question, but let me see if I can explain it in simple terms
Hardware like a modem, videocard, drive, etc needs the CPU's attention to do certain things, particularly interacting with the OS
Modern hardware can act without the CPU in many ways, but this was not always true. Quick example, back when around the 8086 era, a videocard couldn't really draw a box on the screen, but instead provided the hardware to allow the CPU you do it. That meant the video card needed most of the CPUs attention during screen drawing, and other processes had to wait.
Today's gpus will do much of the work and instead alert the cpu it is ready to provide the image already drawn
Anyway, when the old hardware needed the cpu's attention, it would signal the CPU with a HEY YOU, PAY ATTENTION TO ME!" which was known as an interrupt request (you may know as IRQ) .
For the moment, think of an IRQ as a cross road with a police officer in the middle directing traffic. There is a finite number of roads leading into the intersection (say four), and each road consumes all of the officers attention and the other three roads need to stop and wait.
The method of attracting the officers attention was known as an "edge-triggered interrupt", which it accomplished by changing the voltage on the line
This method worked OK, but it had limitations, a key one being that minor fluctuations in voltage due to imperfect circuitry would result in system slowdowns. Another key one was that two devices could not share an IRQ
(we're going back to windows 3.1 and dos). Back then, you would have 13irqs, most of which were consumed by system devices, and you might have 3,5,7,11 and 13 open (or maybe not, depending on system) so you could add 5 things to your PC (a videocard, modem, network interface card, game controller and sound card, for example) and no more
With win95 and up, IRQs were able to be shared, breaking the limitation of how many things you could attach to your computer, but most devices still used edge-triggered interrupts.
Along came level-sensitive interrupts. Unlike edge triggered, whcih signaled by a mere fluctuatioin, of voltage, level sensitive meant that a device had to change the voltage and sustain the same voltage to hold the CPUs attention--meanwhile another device on the IRQ could also use any left over CPU power to do something else.
The clkvl switch does not so much turn anything else on, as turn edge switching off. You will not gain anythng by this, but would simply disable any old devices that rely on edge-triggered interrupts
Now, the interresting thing is, that while any modern componant using a pci slot, agp slot or pci-e slot would use level-sensitive interrupts, there may be motherboard componants that use the old fashioned edge-triggered interrupts.
My only guess on the multi-processor emphasis you are reading about is that such a system is sufficiently modern enough not to use those componants (as would a 64-bit cpu/motherboard combination perhaps)
The point is this switch in boot.ini is usued for troubleshooting...the only thinkg that comes to mind is that it might be useful in checking for a failing capacitor.....or bad motherboard design