Steve - I think the points you made apply more broadly to kit building versus scratch building, not just to the Guillow's B-29. I see a lot of that kind of discussion in regards to model ships and can see both sides of the argument having merit and it comes down to the personal choice of the modeler.
The Guillow's B-29 plans can be had from the AMA plans service, balsa acquired for a song, plastics eliminated or vacuum formed, and decals painted on or printed on new decal paper, but at the end of the day you've gone through a lengthy process that your typical kit builder would find has sucked all of the fun out of it. For them opening a new kit is like Christmas morning all over again while finding and/or making all the stuff that would have come in the kit starts to sound a lot like work.
I have purchased four complete B-29 kits, the cheapest about $85 and the most expensive was closer to $140 because it was sealed in plastic. Yes, you can get a complete Guillow's B-29 kit for under $100 people, but expect to wait and watch carefully 6 months to a year for that opportunity to present, and the months leading up to Christmas are not good months for the buyer on a budget. (start sarcasm) I'm 34 so any of you older than me already know this as it's only the kids these days that don't have patience (end sarcasm).
Since we're talking about the B-29 this might be a good time to remind folks that the first run of B-29 kits had an error in balsa sheets D and E. If your kit has one sheet of D and two of E then you're missing some parts and will have to cut some out of 3/32" stock. If your kit has two D and one E sheet then you've got a later run kit that has been fixed. You can find some pictures and a little more detailed explanation
here and the next two photos after it.