Pros -
Great start to get your music out there
Great way to polish your craft, especially those that produce their own beats, and engineer their own music
Easier and Cheaper than making a full album
As long as the mixtape is non-profitable, you don't have to worry about sample clearance
More creative space
Mixtape covers
Cons -
Sometimes posting your mixtape of mixtape sites (like Datpiff) can do more harm than good. For example, unless you're already a established artist with a following, posting your mixtape of that site will more be promoting the site rather than promoting your own music. The best way to do it is to develop your own website and post them there. That way every thing they see on that site is nothing but you and not other artist. Basic case of Branding brehs
Artist can sometimes get lazy with just making a bunch mixtapes, which could water down the quality of their music. People appreciate originality, making a Cd with a bunch of Pro Tool sessions isn't good for your image. Especially when you're using other artist's productions and beats.