Harald,
You're not completely trapped. You could buy an XXE Pro Edition licence. Or maybe your employer is a bit hard-up at the moment ;)
There is an argument that some IETF work is for the common good. However, who should pay towards the development and upkeep of a good XML editor towards the common good? XMLMind, or the users of XMLMind?
Surely, even an individual IETF contributor can't expect a right to free tools when they choose to make IETF contributions. Similarly, we don't get free flights to attend IETF meetings, or free Internet access because we're working on IETF stuff.
Bob