You are missing out on one thing, a design does not have to be 100% fluid or 100% fixed. There is middle ground. On my site I do something along the lines of width: 47em, with max-width: 90%, and min-width: 20em. This is something that's catching on since 100% fluid on 32" monitors with the browser full screen leads to hard reading. Yea sure, this technique doesn't work on IE since it doesn't support min/max-width, but still the principle is there. Also banners can be made to have some flexibility, depending on the exact banner, there may be some extra part of it that gets hidden with overflow depending on the size of the page. Though again, that's a per basis decision that would need to be made.
You are correct. I was kind of speaking out of experience there. I haven't yet built a site that is a combination. I guess it really does depend on the design and the graphics that you plan for the site.
I suppose the hard part is getting images to expand / contract for different sizes of screens and still look good. Reviewing some of my past sites, I see issues with making my horizontal navs expand, because there would be so much extra room... However, one of my designs that are in development right now could actually be fluid. I think I will make some changes to allow for that and propose it to the customer...
Also, I do not think it is bad practice to use fixed designs. I do think that fluid designs are better when the opportunity to use them arises however, but there is nothing wrong with a fixed design.