pros and cons of PDF1.5?

[Elie]

Hello,

I'm wondering about why pdfTeX (and LuaTeX) are producing PDF1.4 by
default... that's the case of almost all softwares producing PDF
(ghostscript, pdfcreator, OpenOffice.org, Scribus if you don't change
the configuration, etc.) so there must be an obvious reason somewhere,
other than the incompatibility with Adobe Acrobat 5.0 which is totally
outdated... the thing is I don't see it. I only see advantages like my
pdf getting lighter: for a pure text file with an otf font, the pdf
goes from 145176 bytes with the default conf to 123565 bytes (15% gain
which is not nothing) with \pdfobjcompresslevel=3 (accessible only
with \pdfminorversion=5). What do I miss?

What do you think about the idea of making it the default in pdfTeX
and LuaTeX?

Thank you,
--
Elie