Let me take your specific questions in order:
Why abandond the drive in the newest model?
There are a couple of reasons - I should note that I do not agree with them, just stating them. They want to pull more WinDoze 'ultrabook' shoppers over to the Mac line. That's a good thing. The impression is that lighter is better for that market so, they had to lose some weight. Also, Apple obviously believes that 'culturally', we will at some point fail to utilize physical mediums. I can tell you that is never going to happen. Not this year, not in ten years, not in 100 years. Now, before some smartass calls me on my inability to time travel, thus, my answer must be wrong, let me point out the obvious, which is that everything physical has a limit. DVDs do not last forever, but once encoded, they are a solid medium and billions exist. Thumb drives sound great in theory but in practice, despite being able to hold more data, they are no more solid than DVDs and it will be a long, long time before DVDs go away in the marketplace. People want to have physical mediums. Look at the iPad - great for reading books, but every survey shows people prefer actual books. Personally, I do not want to have to have 500 TB of space for all my media, nor do I want a company to have the ability to 'pull the plug' on my media - if it's not physically in my hands, they can do that. Same kind of reason there is opposition to the US healthcare bill; do you feel safer with your information in a folder in an office where the only real fear is fire or safer with your sensitve healthcare information on a drive somewhere where a hacker can wreak havoc? We are simply more comfortable with touch-and-feel.
Why does it make the product better?
It doesn't. IMHO. Now, the Retina screen is something better (I questioned this when I compared it to the anti-glare version of the prior incarnation, see prior post) but the SSD, while tiny, is technically safer and more solid for data (no moving parts, etc). But the lack of a DVD drive (I felt they should have gone the other way and provided at least an option for an internal BluRay Superdrive) does not, by itself, make it better. It makes it lighter, not better. For people who have one computer in their home for business and home use, an external drive will now be a must; the kids will want to watch their movies on roadtrips, etc. In my case, clients want DVDs, so an external is now a must for me, unfortunately.
Does this make the product more efficient?
I can't imagine it does in any way, but I'm not an engineer and, more importantly, I do not have one (yet). However, barring OS issues, loading a DVD vs. loading a movie in iTunes - loading the iTunes movie is faster, but it's obviously not better, since BluRay is much better image quality. Does the speed of the load make it more 'efficient'? That will be argued. I would say it is negligible. It may help airflow not having it in there, it obviously makes it lighter. But that does not automatically equate into more 'efficient'.
You also asked about eco-friendly and frankly, I think these machines are as eco as you can get and can't really be moreseo. MHO. I cannot imagine not having an optical drive makes it (net) any more eco friendly.