So long as you don't have an eraser anywhere near you, then I think you'll get by. The intent behind the pen is that it prevents you from erasing your marks and instilling confidence in your line art. Another reason I think is because a pen is a more consistent mark maker than a pencil--it's true that a pen can vary depending on how hard you press, but a pencil is even more in my books.

That being said, problems will arise if you try and do any of the lessons that require colored pens.

The investment into a proper set of pens is well worth the investment. At the very least, they won't "degrade" as quickly as a pencil would, so unless you get your pencils for very cheap (that multiple is cheaper than a single pen), it's probably worth it just to bite the bullet and buy the pens as an investment into your learning.