The question has come up often, about whether or not Anet would ban for usage of 3rd party API apps, such as gw2stuff's event overlay program. Here is their official answer:
Cliff Spradlin
Lead Programmer wrote:
We have a 3rd party app policy located here: https://forum-en.guildwars2.com/forum/support/account/Policy-3rd-Party-Programs-Multi-Boxing-Macros/ first.
This policy was written before we had APIs available, and needs to be updated to make it clear that we encourage app development and usage. We realize that in its current form it can be hard to figure out what is acceptable.
Our hope is that the new policy will make it very clear what is acceptable, so that you don’t have to worry about getting banned by using an app. The new policy is still being written, and we will update you when it’s ready.
It’s our intent to allow API-derived apps like GW2Stuff’s overlay to be used by players. From our perspective, having an overlay is just a different way of presenting content. We are not currently banning players simply for using overlays, and have no plans to do so.