Namdrol. I agree with your logic. It is faultless. If the protector is harmful it will certainly be taking revenge when one stop practicing. Like a vengeful spirit. On the other hand if nothing happens when one stop propitiating Him then his is compassionate or benign. Ironically, that being the case, then we should then be propitiating HIM since he is compassionate and is therefore enlightened.
On another angle, in a teaching at Drepung Loseling Monastery in Mundgod, India,
http://www.dorjeshugden.com/all-articles/the-controversy/true-harmony-or-a-change-of-tune/, I quote:
They do not understand the situation but just overtly talk like that in Tibet. They don’t understand. If you practice Gyalpo Shugden and can gain enlightenment, I rejoice!
“Sometimes I tease you. In front of Shakyamuni, we don’t pray or wish for long life, or for no obstacles. Do we even know how to pray like that to Shakyamuni? But in the protector chapel we really ask and pray for no obstacles and long life; we pray, “May things go my way”. And at the end of our prayers, we even pray for an end to our enemies! In the temple, in front of Shakyamuni, we fold our hands. But it’s in the protector chapel that we really open up and lay our hopes.
In the above quote, HHDL is saying if one can gain enlightenment from propitiating Dorje Shugden, he rejoices. I read it that he is offering the possibility of that. Protector practice is a supportive practice and if not the main practice for gaining enlightenment. However, in this degenerate age the practice can help lead many from sufferings and to the proper path of practice. In this quote also, he is not objecting to the protector per se but to the practice of many in over relying on the protector for mundane worldly objectives and even to eliminate enemies. That is not the fault of any protector but the fault of humans. (Note that the protector will never grant any wishes that creates negative karma for the practitioner).
My point? Implicitly, he means that since the Protector Dorje Shugden is the most powerful and suitable protector of this age, in that he is the swiftest and grant many wishes, it is also dangerous from the point of view of the practitioner in their over reliance of the protector. Nonetheless, he acknowledges the Protector. I rejoice.