What is the effect of booting with additional cores?

Windows 7 uses what's available on the box ... out of the box. You should not twiddle around with the boot process except when you know what you are doing, otherwise things will become slower / more unstable, etc.

And you should not believe what you read on the Internet.

That said: To tell Windows 7 to NOT use of all the cores that are available is mostly for debugging purposes or some kind of developer tests when you want to simulate a system with only one core. Another reason is to work around buggy installers. Turning the cores off otherwise is not of any other use, the limiting factor of the boot process is mostly disk I/O anyway nowadays (so, buy a SSD when you want a blazingly fast boot .. or don't run that many processes upon startup -> less disk I/O as well).

Again: Windows 7 uses all available cores at boot time. The snippet you read on the Internet is most likely the super-über-hack to go into MSConfig and then check the Number of processors box and then click the number of cores you want Windows 7 to use. People writing about 'how to speed up the boot process' did not understand the what that checkbox is used for:

Number of processors. Limits the number of processors used on a multiprocessor system. If the check box is selected, the system boots using only the number of processors in the drop-down list.

It LIMITS (as in 'puts an upper limit of used cores') the boot process to take only UP TO processors and (this is also important) later on provide only that amount of processors for the OS. If you select one there you will run a system with only one processor.