Sure. And, in fact, I've stuck things like exim and pine versions in from
source on Debian before. But the whole point of having a tool like apt in
the first place is that you should be able to use it to install software
automatically for you, and it's likely that some percentage of users is
going to be uncomfortable with installing things from source (that being
the segment of the userbase that such tools are trying to reach). Plus
there's the support issue which other folks have brought up - if you're
paying Redhat Corp some number of medium-to-large size bucks to look at
your machine for you when it explodes, you don't want them to shake their
heads in disapproval and blame your non-standard OpenLDAP source version
(which they will happily do whether it has anything to do with the
problem or not). And, of course, you now are allocating extra storage for
no good reason, building new things which rely upon outdated libraries
(that is, the other packages), instituting two upgrade procedures instead
of one to account for your source version, etc.
I guess what I'm saying is that the availability of source versions does
not remove the obligation of package maintainers to keep things up to
date (obviously this particular bit applies less to Debian, since the
maintainers are volunteers). And that people should be open to
investigating alternate distributions which may be less... mmm...
problematic in general, rather than patching over the problems in
existing distributions with source.