As far as showers for second babies go I think they are not necessary. The purpose of a baby shower is to "shower" the mother and father with gifts for the baby that they need... and usually by the time the second baby comes around, parents are all set with supplies.
As far as showers for second marriages... I think that it really depends on the situation. Sometimes, when you go to a wedding or wedding shower, you can tell right away if the marriage will last or not (I know that personally, I can think of 2 different couples that fall under this category). Actually, you can probably even tell that BEFORE going to the wedding or the shower. In the case of the shower mentioned in this thread, I think that is inapproperate... and not cause it's a 2nd marriage, but basically just because of the theme.
This is all just my opinion though.
