Re: What are the requirements for a messaging app?
Well, WhatsApp has a Desktop client that means I no longer have to use the Web page, which mean I don't have to worry about the web page always being open - the app starts with Windows and is always there, yay!
Just a few issues though:
1. Your 'phone *has* to be on for messages to appear, not terrible but not great.
2. Messages ping the phone and the PC.
3. It doesn't support SMS messages, so you need to both look at your phone and work out the message history on your head where mixing has occurred.
And before you ask why someone would use a messaging service and text messages to the same person, perhaps switching during a chat, well it happens all the time based on coverage and whim.
Skype may be despised by some but it offers merging of messages and SMS, on the PC and the 'phone, tying the person concerned into one 'Contact'. It transfers the messages to all targets and notifies on all targets but, crucially, once one starts chatting on one of them, the others stay quiet unless a period of time has passed without messages.
So, get a text? See it on your PC and answer it right there, super useful, without any extra App on either device.
Well, that's what happens on my 'phone/PC anyway.
I have WhatsApp because mobile-orientated people swear by it, which is fine, I don't have to use it much at all these days and the phone App I use has no ads or serious horribleness.