My situation:
Took up a job that I was not looking for(didn't really think I would enjoy it and that I would have to 'bear it' for 18 months, but it satisfied some of my other criteria including my wanting experience in a very large company so that I could observe the culture) (the 18 month thing is the min duration at a job before one can change jobs at this company)
Opinion:
Without alluding that I am better of than if I had gone with what I wanted (project engineering/product development - in line with my previous experience) since I will never that know for a fact, it has been a great learning experience.
The things that I learnt, that I had to do at this job were completely different from what I had done previously, and exactly for that reason it has increased the breadth of my skill set in a very good way. Now, I consider them as invaluable additions in my attempts to achieve my future goals.
Companies spend massive amounts to know what the customers want, domain experts are forced to interface with customers so that they can use the feedback in designing better products or systems.
Your new role can be a treasure trove if you use this opportunity to get inside the customers' minds and give you that added edge if you were to get back into systems design later.
About the 'job responsibility' that you mentioned, this would be my suggestion for you:
By all means, use the fact that you were given responsibilities that you didn't apply for and were not looking for as a negotiating point for better assignments, favors or whatever..
But, if you really try to limit yourself to 'job responsibilities' you will be shorting yourself, closing yourself to new learning experiences that can actually enrich your knowledge base.
Dive into it and learn all you can. That is, if you really can't change the jobs and get what you want.
Good luck. (Make lemonade when life gives you a lemon :))