IM2000 is even suited to off-line readers, too.

M> In case of IM2000, it seems the recipient needs to maintain 
M> network connection while he is trying to decide which email
M> actually to see.

No.  The decision can be made offline.  One only has to be on-line whilst one
is actually performing the transactions that one has decided to perform.  

Read the case study of Lucy reading her mail.  Notice that the recipient MUA
doesn't hold any sessions open in between user actions.  Lucy could take a
day, or a week, to read the list of notifications and decide what action to
take next.  (And although the case study doesn't have one, if there were an
intermediate stage of fetching "summary" information about messages whose
notifications piqued her interest, she could take a day, or a week, to read
the "summary" information too.)  She doesn't have to be on-line in order to
ponder her decisions.

Indeed, operations can be batched.  One can imagine an IM2000 recipient MUA
tailored for off-line users whose user interface allowed the user to highlight
selected notifications from the list of "new" messages, and then invoke a
"fetch the 'summary' information for the highlighted messages" action, for
example.

An IM2000 recipient MUA doesn't have to work exactly like an IMAP MUA does. 
Although there are similarities in the notion of client-server access to
message stores, it's important to remember that there _are_ differences.