[exim] -qq vs. -q

Hello,

I am considering switching my main mail server from Postfix to Exim 4.
I have been doing a lot of reading about Exim and have installed it on a
few small systems.

My main questions right now revolve around the queue. First off, the
difference between -q and -qq. According to my reading of the manpage,
-qq would be quite desirable -- it seems like it would go easier on both
local and remote resources by establishing a single connection to a
remote machine for transmitting various different messages that are
going to it. Yet -qq is not the default. Why is that? It makes me
think that there is a tradeoff that I'm not understanding. Is there?

Also, from my reading of Exim's queue-management system, it sounds a lot
like the Sendmail queue manager I remember from my days prior to Postfix
(say, 5 years ago). That is, it could get hung up repeatedly on sites
that are extremely slow or have nonresponsive DNS servers. Is that
really the case? Also, Postfix has a lot of controls regarding backoff,
ramp up, etc. that seem useful. Does Exim have them somewhere that I've
missed, or is there some design feature about Exim that makes all of
that unnecessary?

Thanks!

-- John