1] For a day-to-day changing alpha release it makes plenty of sense to
keep the initial download as small as possible, since so much is
expected to change as part of the development process.

2] OTOH, a developer wants to encourage people to test the install and
the release often, so it makes sense to have an initial iso download
packed with the stable and large software packages that aren't central
to the what the distribution is innovating. Any time a user runs a
second test, she incurs a bandwidth burden of an entire new install.

3] One complicated solution would be to not destroy
/var/cache/apt/archive on the target when re-installing. It could be
done by having the installer suggest to mount that folder on its own
partition, and then have the installer refer to it at the download stage.

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