IMP needs a db for contacts, preferences, and sessions.
While it's *technically* possible to store sessions in shared
memory, it's sort of a "gray area" with IMP, as it's not
used very heavily, none of the core developers use shared
memory sessions themselves, and there's not a huge body of
knowledge on the mailing lists in terms of configuring and
debugging it.
It's essentially a PHPlib feature that wasn't explicitly
taken *out* of Horde's modified PHPlib, but it's still
essentially deprecated.
For practical purposes, IMP 2.2.x /does/ need a database for
sessions.
-Rich