Your 1 is not what Additive Synthesis is.. what you describe is just 'adding more synths'.
Additive synthesis is specific synthesis technique where the sound is built out so called 'partials', which basically means a whole mess of sinewaves being combined to form the sound.
There aren't that many of those synths, because it gets quite complex and it's really heavy on CPU power to calculate enough partials to make thick sounds.
NI's Razor is an example of it, even though that was designed to kind of resemble how you operate a subtractive synth. It has filters and reverb, but they don't work like traditional effects, they modify the actual partials.
It's pretty interesting and gives some pretty unique sounds.
I myself hardly ever use layering actually. Except maybe when it comes to designing my drums.
Usually there's other, more practical ways to get a similar effects. A big reason is that I really hate when projects expand like crazy. Everytime you double something,
it means an extra hit on my CPU, an extra mixer channel to manage. If you bus, that's three channels instead of one. For my workflow that really does not work at all.. I haven't had complaints about my stuff sounding thin or weak... lol, more the opposite.