What, are you not a good boy?
just kidding
Ok, in seriousness:
Skype lets YOU limit who you talk to. No one can gain access to your personal information unless you A: Put it on Skype, for whatever reason,
AND B: Add them as a contact
As long as you are careful of who you add as a contact, you won't run any risks. Skype itself is a very clean program with currently over 49 million people online as I'm writing this - and that's not even including people who aren't online right now.
Also, the advertisements are very small and don't shove themselves in your face, and they themselves are clean of viruses, malware, and pretty PG-rated. (Or E for Everyone? Idk.) Heck, there's not even ads 100% of the time.
Skype is useful for communication - both with online friends and your own family! Your parents, siblings, real life friends, and relatives who may have it can use it to get in touch with you if you're on your computer and/or phone. Unless you have a cell phone or something, then it's kind of meh. Also, if you're unable to attend family gatherings, you can still do video call for free, unless you don't have a webcam, they don't have a computer handy, etc. (also it's only with 2 people, not group video, which does cost extra.)
