In regards to going to just part of a seminar...
I can see the point of someone might miss out on what is being taught previously, and I have seen this in seminars as well. But it's not that hard to figure out what is going on and pick up on the theme. I can see it possibly being offensive if in order to do the techniques on Sunday you have to have been there Saturday. From what I have experience partial attendance is accepted, encouraged, and allowed, there really isn't a continuity issue. If partial attendance was offensive to the particular sensei, assumdely, he or she wouldn't hesistate to request that people sign up for the whole seminar. (Of course, I'm sure he or she would be diplomatic about it.)
Where partial attendances is allowed and accepted, someone who has tight finances can easily choose the one day where he can get the most for his money. It's better to be there one day than to not be there at all. It is a great option when it is permitted.