I'm part of several Skype groups from another forum, and it definitely has its benefits, as well as drawbacks.
On the negative side of things, it's true that it can add to a mob-like mentality where people find strength in numbers. For example, I can't tell you how many times people who have been infracted or banned for whatever reason, show up in some Skype group, retell events in a skewed or untrue manner, and then has the support of a large group of members who then take their upset to the actual forum. Thing is, though, I honestly think this is more of a human nature thing, rather than Skype itself--when people are feel they have been slighted, they seek refuge from like-minded people, friends, acquaintances, whatever, so that they can feel better about what's happened. Truthfully, this isn't really any different from when IRC was more A Thing back in the day, and it's shifted more to Skype now. But yeah, joining a Skype group can get that way should things go sour.
It can also be a place where, when people are gathered as a group, gossip and malicious chit-chat can take place because, as I've said prior, people take comfort in having strength through numbers, so the nastier side of personalities can show when people feel they've got a solid group who has their back.
Also, as others have mentioned, there's a greater chance for the exacerbation of conflict which, again, can be attributed to human nature in that people are without censor or rules, and communication is instantaneous, can clash quickly and in a very ugly manner, whereas with posts on a forum, people (theoretically lol) can think before something is said/posted.
There have been issues of information "leaking" from private chats, and I've known it to cause much trouble among circles of friends.
I've unfortunately seen all of the above happen more than once over the years in regards to Skype chats and forum groups taking their business away from the forum, and I'd like to say I can just ignore it all except a lot of it returns to the forum where I mod lol, so it can be a pain in the *** to sort.
All in all, a lot of the -negatives- I'm listing really comes down to how people often react in group situations, which can be an online or offline thing so I wouldn't count it as something that's especially exclusive to Skype as Skype is just a tool, really, and this can happen almost anywhere there's a group where diverse personalities can clash in an immediate nature. Some things, like the leaking of logs and such, can be exclusive to chats.
---
That said! There are also positives to it:
I've seen some amazing community-building and planning doing in such groups. An example of this is when there are large-scale community competitions, and I've seen people get together on such programmes like Skype, encourage each other, share ideas, and just really let team work flourish over the medium.
I've seen bonds that have formed that have eventually crossed over into RL, believe it or not! In fact, my best friend is someone I prob would never have gotten as close to as I have now, were it not for a certain Skype group. I've traveled to Canada more than once, met up with him, hung out for days, and it's become something of a yearly tradition now. I'm actually going back to see him in January.
I've seen people from different parts of the forum (for example, someone who only posts in the Zelda section, and someone who only posts in the Entertainment Section), cross paths and become steady friends as they would not have otherwise as their interests do not align on the forum.
Fun group stuff can happen too like I've had voice calls with Skype groups while we all watched a movie, or played Mario Kart, or Smash, or whatever. It can add a fun interactive dynamic.
Maybe not for me so much, but I have seen it be something of a comfort zone for some people who turn to the group when they are feeling down. This goes hand in hand with the negative, actually, that I mentioned in regards to people using it to vent frustrations and stirring up a mob, but there are times where I've seen people use it to vent about RL stress, and it's actually helped them.
---
For me, personally, before I'm added to a group, I ask
-Who is already part of the group so if there is a name of someone I'm prob gonna not get along with, I will decline (or names of people I'm interested in getting to know, in which case, I may accept!)
-How busy the chat is because thing is I'm not too great with super-fast moving chats lol
-Does the group delve into much gossip. If the answer is yes or kinda, I'm likely to decline as I tend to steer clear of that
I am very picky about which groups I join, and I will be quick to exit if I think the group is deviating to a path that I find uncomfortable, because I, frankly, use the internet as an escapism from RL stress, and the good thing about contact lists is I can control who I talk to and who I don't talk to, so I make sure that that experience is kept to a positive one, for the most part. Further still, I -mod- elsewhere so I get enough complaints and anger and tantrums and fights I have to sort there lol, so no way I wanna cross that over to my IM stuff.
Do I think it's worthwhile joining a Skype chat? Sure~ because I do think it's a great way to find people you may not otherwise speak to on a forum. It's happened with me, and for all the bull**** and dumb stuff I've seen, finding that one person I call my Partner In Crime now was absolutely worth it. It's def. not without its issues, joining Skype chats, but it most certainly has benefits too
(Note: I did not mention one-on-one convos over Skype in any manner because I do not add people as contacts unless I know them well enough to feel comfortable with it, so all my one-on-one convos and interactions are good ones as I make sure of that)