@Tuoni
i like the ability to chat to guild mates whether in game or not. If Discord was used then I could be in chat all the time via mobile, pc or in game. to me that's a great system. Sure there would be limitations, like item links but i believe those could be structured like HTML links so that clicking it would do the normal popup in game or open your browser to the item's DB page.
imagine getting home and seeing on your mobile that an attack will happen soon. you know then that you need to get on by x time so you can do somethings prior to the attack. if it was in game only, then you'd get a notification via discord on your phone on the events happening in game and that you need to be online now.
i played a game where the group (it was a 4-5 player coop arena match) was put in a discord voice chat to speak to each other. we could use the text chat but no one did. to me that was brilliant, let the company that specializes in communication do their thing and you just use parts of it. that's probably cheaper than developing your own voice chat system.