Starting with something light and making eye candy available as an option sounds better than having extreme eye candy (gnome3, kde, Unity) and having to endure extreme slowness to get to lxde or openbox.
I don't see why people are making a big deal out of the change. The net installer already has an option to choose between LXDE, Gnome, KDE, and XFCE. I'd suspect anybody installing Debian themselves and who has a preference would know how to change between them.
That said, it wouldn't hurt to make the option could be a little easier to find.
When running on old hardware offer something light (all the way down to text, say for mainframes).
After using lxde (lubuntu) for about six months, the Windows desktops and the fancier Linux desktops now look like the red light district with prostitutes on display in flashing light-lined windows.
But in the long run, a default desktop will be more thoroughly tested than a non-default. This is less true for debian than ubuntu, but I'd guess there's still some effect.
Then there are other packages which will never be less tested than a default because they have already been tested for a few years.
It's not GNOME's fault though -- it's an annoying issue with big metapackages. Same thing happened with XFCE on a stock Ubuntu install.
That's not true. The "default" gets much more attention from users, and most new users will use it.
In a project like Ubuntu, for example, the "default" gets more funding, more people working on it, including professionally and more tailored themes.
Choosing to install a particular desktop environment when you want a "graphical environment" is only a matter of convenience : proposing GNOME from a CD install is getting harder because of the size of new versions. Several ideas were proposed : changing the default DE in tasksel, changing the compression scheme for .debs (using LZMA instead of bzip2), or even dropping CD installations IIRC (keeping just CD and USB netinstalls, and DVD installations).