Question: Is it morally or spiritually wrong for Christian to have seders? Answer: No, but it's weird for a number of reasons.
I've actually participated in seders with Jewish friends back in Oregon and I don't think it's wrong to do so. As I see it, it's like a Danish friend coming to a 4th of July celebration when in the US. For the Jews, like Americans, Passover is 4th of July. So, just as we have a national history and a story of liberation, so do the Jews. The fact of the new testament doesn't do away with Jewish identity or national history, any more than it does away with let's say, Russian, or German or Indian history and national pride. So if Jewish Christian want to celebrate Passover as a time of remembering national pride and liberation, the way American do for 4th of July, that's fine. Just as long as they don't think that it is some sort of alternative means of salvation or something which they must obey as a fulfillment of the law.
This bring us to the second point. Of course part of the difference is that these events and the meal points ahead to Christ and the institution of the new testament in the form of Lord's Supper as well. Here's where it gets weird for Gentile Christians.
Having seders for Gentiles Christian on their own (if not invited by Jewish friends, as in my case) doesn't make any sense. My ancestors were Swiss/Swabian German immigrants in southern Russia who later settled in North Dakota. As I recall, Swabia, Switzerland, Russia, and North Dakota are nowhere close to Egypt or Israel. So, why would I celebrate the national liberation of a people I have nothing to do with? It would be like the Japanese celebrating 4th of July!
Secondly, Gentile Christians don't need to celebrate seders because they have Christ and his supper: "Christ our Pasch has been sacrificed, so let us keep the feast." The Passover Lamb was the shadow, and Christ the reality. So, as the author of Hebrew puts it: What's the point of going back to the shadow, when you've got the reality? Answer: None.