Modern XC Geometry - Whats in it for me?

I'm firmly in the XC camp, mostly endurance riding rather than hucking jumps and riding really aggressively.  From what I have analysed from looking at geometry charts for various bikes is that the geometry of 29er bikes is still evolving, with geometry being specific to the bikes purpose.

Longer (up to 450mm) chainstays help hugely with seated climbs where you can spin up the hill (rather than blow your energy out of the seat), and longer front centers (bigger reach) helps with out of seat climbing, and have a larger sweet spot for weight balance between the front and rear wheels.

I believe that chainstay lengths for a more playful bike need to be shorter (my old 26" Kona HT had 409mm chainstays! but looping out on climbs was a common occurance. ) but its a trade off.

Giant Fathom 2017-2018 Geometry Chart

The evolution can be seen with some manufacturers, see Norco's latest range, here is the Norco XC race geometry (note the chainstay length is increased from sizes S-XL), with an emphasis on climbing (where races are won of course!).  I think this is where geometry and wheel sizes for XC are going..

Norco XC Race HT geo 2018

And look at a 'trail' orientated geo (26", 27.5+ )... on the Norco Fluid HT below.

Norco Fluid HT Geo 2018

Longer front centres, spec'd with 60mm stem, shorter chainstays, but still changing from XS-XL,  and bringing back 26" wheels on small sizes !

From MTBR forum, some posters had these thoughts :

The real values of interest are the BB to seat rail center measurement, and the ETT-Reach (horizontal distance rearward behind the bottom bracket the seat rail center is)... the angle derived from taking the arcsin of those will be the actual user STA, but those all vary by individual BB-Rails measurements (variable for droppers) and where the rider puts the seat... so the STA from a GEO sheet is nice to know, but at most a fraction of the story, and the variation between the ones posted is really unhelpful.

To get tighter chainstays, a non-linear seat tube and/or aft mounted bottom bracket is usually the immediate answer, so you'll get an 'effective' STA, and cumulative STA (which is accurate at one particular seat post extension amount - and doesn't take into account fore/aft seat travel). This is why the seat tubes will be relatively slack on a lot of 29ers, or longer travel bikes. If you're willing to run the seat slammed forward, or even try things like running a layback seatpost inverted (can be done to put the seat even farther forward) if you really need to be more over the bottom bracket.
I realize the issue you'll have if you're mostly taller from the lower body is that those slack effective STA bikes is that the seatpost extension to get out to your BB-Rail height is putting you even farther back... that said, being over the rear tire isn't all bad when climbing, especially if it's paired with a longer top tube so that you can still very easily weight the front end of the bike - if anything, this initially strange and rearward biased weight distribution for boring climbs can actually mean better climbing ability when it's traction limited - and longer ETT bikes mean it's easier to shift your mass to be in that sweet spot. The light front end (when I'm at the back of the seat) is actually pretty nice in a lot of instances, as long as scooting slightly towards the nose of the seat can re-plant the front it works great.

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I have pretty much changed all my bikes to forward geo (XC, trail/enduro and fatbike) and for me that has resulted in only positive improvements in both climbing and seated peddling. Moving the saddle forwards via either a steeper seat tube or simply having the space between the bars to push it further forward on the rails to put hips further over the BB has resulted in a much more efficient peddling platform for me with huge improvements in lower back discomfort ( I have a bulging disc which causes sciatic issues).
On my longer slacker enduro bike I can climb steeper trails than my friend on his shorter XC bike because the longer bike aids both rear wheel traction and prevents front end lift. Sure it's heavier and not as peddle friendly as his XC rig but I'm still riding long after he's looped out and pushing.
Also as mentioned above, a longer bike with a shorter stem allows a more centred riding position which definitely helps with off road body position. 
Whilst I fully appreciate you can achieve the same seated reach/ BB to bars measurements on a shorter bike by playing with stem lengths etc, I find that as soon as I venture off road on a shorter bike with longer stem I end up having to put my body in such a position to move my weight further back to prevent OTB scenarios that it creates muscle tension.
On a forward geo style bike I can have a more comfortable riding position with a neutral stance when standing with out the OTB feeling which allows you to be so much more relaxed on the bike.
There is a lot more going on than simply increasing reach but for me, moving to slacker head angles, longer bikes and a stem no longer than 40mm has been nothing short of a revelation in terms of both comfort and performance. The idea that longer bikes don't go round corners is a bit of a myth too in my experience. 
There seems to be an awful lot of people ready to bash the forward geo concept, a few of my friends included who thought "that'll handle like a truck" or a "I prefer a shorter more playful bike". Whilst I appreciate it may not be for everyone, variety is the spice of life etc, I would urge anyone curious about trying a longer bike to keep an open mind and try an extended demo on one ( a couple of days). You may me pleasantly surprised."