Thanks everyone for your time (or rather sorry for having wasted it).
Just in case it's interesting to you... The reason we implemented things this way is in order to avoid a deadlock situation - if we send two queries as P1/D1/B1/E1/P2/D2/B2/E2, and the first query has a large resultset, PostgreSQL may block writing the resultset, since Npgsql isn't reading it at that point. Npgsql on its part may get stuck writing the second query (if it's big enough) since PostgreSQL isn't reading on its end (thanks to Emil Lenngren for pointing this out originally).
Of course this isn't an excuse for anything, we're looking into ways of solving this problem differently in our driver implementation.
Shay