well i've heard of 2nd baby showers to have a diaper theme. so they just bring you the necessities!
I agree with the others - it depends on your situation and how you feel about it. My sis didn't have one for her second b/c they were under 2 years apart and she figured that anyone who would want to buy for her would buy anyways, that they didn't need an invite to buy for her, but I have attended showers for those who were having their second child and it was fine too.
I think that it depends on the situation....For example....if your last child is alot older, you probably got rid of alot of stuff and need new items......
Last year I had a shower for my friend b/c with their 5 year old, they went thru 3 rouns of IVF before she got pregnant..and this time around she got preggo the old fashioned way...never thinking that she could. So they thought that they were just having the one, and got rid of EVERYTHING....and people were more than happy to attend b/c they were so happy for her!
I had one for all of my pg. I didn't throw them or ask for it, friends and family always did. In my opinion, it would be up to you. Everyone has there own opinions on it. Good luck.