IMO there are no disadvantages of having a dual or multi-OS boot option.
The advantages should be pretty obvious - having the option to choose your OS based on your requirements or preferences for certain tasks.
the advantage to Windows, is that nearly everything works. Nearly all hardware has drivers, all software is compatible.
If you install Ubuntu make sure that all of your hardware works (has drivers) for that OS.
Especially your video card, and motherboard.
I would install Ubuntu on another HDD, not just another partition.
Lastly, as a word of caution, i've seen so many people get rid of Linux by just reformatting the HD/Partition that its on. THIS IS A BAD IDEA. the computer gets angry, and you have to end up rebuilding the boot file which believe me is no fun.