Allow me my small amount of hyperbole in the title. 
Skype uses way more resources than a simple chat client, or even a video chat client, should be using. (Even if you are comfortable with video chat using up tons of resources because that used to be normal for video apps 2 years ago, Skype shouldn't be doing that when you're not even in a call, but it does.) This is in part because it now serves you high-def, lag-free, obtrusive advertising alongside its laggy voice/video.
Somewhat-recent Skype problems:
The UI is so bad and wasteful, people are downgrading to get their screen back.
Skype is popular enough that it's frequently used to find a DDOS target.
The Android version of skype can be tricked into calling you back.
Every time Microsoft fixes something, they're updating their bloated version of some other company's spaghetti code. As soon as they do it, another vulnerability is found. It's a popular target because it's a popular piece of software frequently found on the laptops of teenagers and grandmothers who are more interested in the conveniences of always-on social apps than practicing good security culture. Microsoft is so far on the back foot that, so long as Skype is running, you may as well have "Enable remote assistance" turned on in your router's settings.
IRC is tried and true, it doesn't load a million lines of backchat when you log in, it uses an obscenely low amount of resources. It's not popular with grandmas, which means it's not a lucrative target for hackers. You can use whatever client you want, but I recommend Hexchat.
Or for something more modern: any client that uses XMPP can talk to any other client.
If it supports XMPP Jingle, you can do video/voice too. You are probably most familiar with Google Talk or Jabber as far as XMPP clients go. Can Skype talk to other clients? No, it's proprietary and closed source. If Riptide Lab were to use XMPP, folks could use Talk or Jabber for the no frills option. Nublets would be free to download the latest "free""popular" sponsored client from some Russian guy the Google Play store or the iWhatever store on their walled privacy nightmare garden mobile device. And I'd be free to use something good like Gajim. And we could all talk together!
But seriously IRC just works.
When we did our first cockatrice cube draft a couple weeks back, Eric was moaning about what a pain in the ass Skype is... don't recall the exact deets but maybe he can remind us. Here's my take on it:
Skype uses way more resources than a simple chat client, or even a video chat client, should be using. (Even if you are comfortable with video chat using up tons of resources because that used to be normal for video apps 2 years ago, Skype shouldn't be doing that when you're not even in a call, but it does.) This is in part because it now serves you high-def, lag-free, obtrusive advertising alongside its laggy voice/video.
Somewhat-recent Skype problems:
The UI is so bad and wasteful, people are downgrading to get their screen back.
Skype is popular enough that it's frequently used to find a DDOS target.
The Android version of skype can be tricked into calling you back.
Every time Microsoft fixes something, they're updating their bloated version of some other company's spaghetti code. As soon as they do it, another vulnerability is found. It's a popular target because it's a popular piece of software frequently found on the laptops of teenagers and grandmothers who are more interested in the conveniences of always-on social apps than practicing good security culture. Microsoft is so far on the back foot that, so long as Skype is running, you may as well have "Enable remote assistance" turned on in your router's settings.
IRC is tried and true, it doesn't load a million lines of backchat when you log in, it uses an obscenely low amount of resources. It's not popular with grandmas, which means it's not a lucrative target for hackers. You can use whatever client you want, but I recommend Hexchat.
Or for something more modern: any client that uses XMPP can talk to any other client.
If it supports XMPP Jingle, you can do video/voice too. You are probably most familiar with Google Talk or Jabber as far as XMPP clients go. Can Skype talk to other clients? No, it's proprietary and closed source. If Riptide Lab were to use XMPP, folks could use Talk or Jabber for the no frills option. Nublets would be free to download the latest "free"
But seriously IRC just works.
