Why use Threema when there are alternatives that are not closed-source? You had to begin to use Threema, which presumably carries the same difficulty as beginning to use something which isn't as questionable.
Telegram for example doesn't exactly tick the "not as questionable" box for me, not least because it isn't entirely open source either, but also because I always find it weird if someone is pouring money into software that is available for free, with no obvious benefit for them in return.