IMHO:
It's a valid question ... one I've been wondering about for many, many years as I watch hundreds of homecrafters build their teardrops that way.
If you look back in history to the most famous, most successful, highest-number-produced teardrops - from
Kaycraft Kampster to
KIT Kamper to
Ken-Skill to
Benroy to
Serro-Scotty to
Scad-a-bout and so many others that have survived the test of time and the torture of the road,
none of them had built-up floors, just simple plywood bolted to the steel frame.
Yes, a couple of the early vintage magazine plans showed built-up floors, but you'd think that by the year 2000 we would have progressed beyond that! Commercial manufacturers certainly did ... over 60 years prior! Believe me, I'm not saying that the trailer industry of the times got everything right or should have all of their methods held up as examples to follow (hell,
NONE of them expected their products to last 25 years, let alone 50 or 60!), what I am saying is that many of them got some things simple, efficient, straight-forward, and just plain right! And, remember ... we're talking
teardrops here, not
RVs ...
Nostalgia is one thing ... and so is doing something a certain way to achieve a certain "woodwrightsmanship" ... but if your goal is to build a solid, long-lasting, good-looking teardrop as efficiently and inexpensively as you can, why
would you spend the extra time & money on a built-up floor when a simple piece of plywood does the job so much more adequately? Some of us
can cut a straight line with a handsaw, but today there are power tools that do the job so much easier and faster and cleaner and ... well, you get the point ...

...
So, I'm going to be watching this thread, too, to see what you all have to say! I've never claimed to know it all, nor am I too old to learn anything new (or even not-so-new ...), but I'm always willing to be surprised!
CHEERS!
Grant