As for Discord... my guild leader switched our raid team from Teamspeak and there was some acrimony because a guild member refused to install Discord as he felt strongly that it was a security threat and malware.
I did try to substantiate these claims but couldn't find proof one way or the other.
As a precaution I've resized my hard drive and installed a separate Windows for gaming and encrypted each partion separately to isolate Discord from my personal files.
Several weeks into using Discord I think it is better than Teamspeak for overall features but in a raid situation with 20 on simultaneously the quality isn't as good as paying for a private Teamspeak server. Having said that though Discord is free and allows for a much richer experience so if they can sort out voice reliability and quality it will be a compelling proposition.
I still have concerns as to the security of Discord as their website pretty much brushes over the topic but they do offer two factor for what it's worth... but I can't raid with my guild without it so I'm stuck using it.
(I assume you don't do this for every application you install)
a) vendors I trust b) confident in their security, and c) couldn't write the software myself.
In the case of the Curse client I just wrote my own batch scripts and used cygwin. I could have just as easily done it in Python or AutoIt.
As I wrote the software I know there is virtually no chance for malware.
I can't be as certain about the Curse client. As far as I know it isn't open source.
Maybe I'm more paranoid than most about security but it could be because I know how easily most systems are compromised.