For me personally the biggest pro, by far, is as a cousin finder! I have made connections with several "cousins" or otherwise connected family researchers. They have surfaced in Norway ( 4 all with new information I never would have discovered on my own), Germany( 2 from my husbands line and from my paternal grandmother's line), and Ireland ( 2 from my maternal grandmother's line), in addition to California, Texas, and Illinois. I am sure that between my on-line trees and this blog I will find other connections. Another pro would be that on someone else's on-line tree, I found that my husband's fifth great-grandfather was a Revolutionary War Patriot. I was able to compare the tree with what I had to ascertain the connection and I already had courthouse copies of corroborating evidence in the form of wills linking the third and fourth great-grandfather's to more recent family members. The Patriot's will has also been posted on-line making an application to the DAR easier to document.
Probably the biggest con for most genealogists is the fear of having your hard work "grafted" to someone else's tree or even posted and renamed as their tree. It happens! It happened to me but I don't post all of my sources on-line although I will share them if contacted. I know I did the work and I know it is correct although it is still subject to change as I make new discoveries. It is sad to know that someone has taken my work and posted it as their own but frankly I like that by posting on-line at ancestry.com and My Heritage.com are out there working on finding new "shakey leaves" and "record matches" for me. If my trees were not out there, they couldn't work for me!
Please, please, if you choose to put your tree on-line be sure to use the privacy protection provided by your program choice and protect any information of living people.
images: google images