Living on the proposed southern route of the extension, I've never quite been
able to work out why it's been trumpted as so important. It doesn't open up
many (if any, really) new journey opportunities that seem likely to have any
great demand. All of the southern stations are currently commuter stations
to London Bridge and London Victoria, and unsurprisingly commuter journeys
to those terminuses are the vast majority of journeys. I can't really see
the demand changing from that and all of a sudden there being a rush of
people travelling from, say, Forest Hill to Whitechapel. Which is a journey,
like most others using the proposed extension, which can already be made
easily with just one change anyway.

but surely the ELLX does the following?

a) extend Tube services to South London

Not in a meaningful way. For any sensible journey, it would simply
move a change at London Bridge to a change at Whitechapel. It would be
better to scrap the Northern half of the project, abandon Shoreditch
and extend the services terminating at Tower Hill and Whitechapel to
the South Central (then there would be serious new journey
opporunities - a one seat ride to the middle of things at Charing X
Embankment and losing one connection from any journeys involving the
ECML, MML, WCML, or GWML.

b) provides a new cross London service

No, it provides a service to stations in North London that aren't even
on any main line. One of the major reasons for Thameslink being so
successful is that passengers on the Midland Main Line have a single
change to get to the South Coast.

c) provides faster journeys to Docklands via Canada Water / Shadwell
stations

Depends where you're starting from. From Clapham Junction, it'd be
quicker via Waterloo; from Croydon, it'd be quicker on a fast train to
London Bridge; from New Cross you might as well walk to Deptford
Bridge. In fact a far easier and more useful extension for South
London to Docklands travel would be a DLR branch over the New Cross Rd
with stations at New Cross, New Cross Gate, and Queen's Rd Peckham.

d) provides a new access point to part of the City via the Shoreditch
High St station

It'd be almost as much a walk as from Whitechapel.

e) provides a Tube service into the London Borough of Hackney

Here's the real motive... pity it doesn't go anywhere like Chelney was
meant to.

f) provides a potential for economic development in deprived parts of
London

Explain.

g) provides an orbital rail service across South and Inner East London

Not quite. Tangential more like.

h) provides a second Tube link into the Tramlink network

And like the existing tube link, it has little practical use. Just as
from Wimbledon, you'd ride into Waterloo unless you wanted
specifically to go to Pad, I'm pretty sure anyone from Croydon would
get on a Fast train to Victoria or a Thameslink rather than sit on a
slow Underground train to Hackney (unless they wanted to go to
Hackney.

i) potentially reduces the loading / congestion via key Central London
termini and their adjacent tube stations.

This would be particularly useful at London Bridge, but ELLX in its
current form looks like a bit of a damp squib.