Good luck...i guess...

ULs are essentially range toys, it should be fine so long as I don't use the gun to buttstroke any zombies. If it breaks, "oh well", swap a replacement in there. No big thing.

I thought the intent of a lightweight build was to make it easier to carry around all day?
I for one haven't built a non-SBR AR in years, so for me i'll stick to something that I know will not fall apart one day...

I would agree that if you build a plastic gun...it is a range toy.

It's not just that, new shooters (especially women) are a lot more comfortable manipulating a 5.5lb gun compared to a 8.5lb gun.

That could be one purpose, but the ultralight crowd is involved in lots of different things. A guy using a poly lower isn't likely building a battle rifle. Some people derive pleasure from building things with different design objectives other than SHTF. The comp crowd is a subset of the ultralight crowd. They are the driver behind aluminum/Ti carriers, Ti gas blocks, CF (or otherwise ultralight) rail systems, and the like. Most people in the LW crowd don't just pick the absolute lightest option for every part, they tune the build for their needs. So if YOUR objective was a 'fighting gun' that needed to be light so you could low-ready it for hours at a time, maybe all of your parts forward of the receiver would be super light stuff. You could do that without compromising durability much at all and you'd prolly use a mil-spec type lower, not a poly one. Just because you're not an ultralight builder doesn't mean you can't take advantage of select parts that have been designed/built for that niche.

As far as SBRs go, "me too", my current ultralight build project is 'both'. 10.5" Faxon pencil bbl, AL thread protector, brigand arms CF handguard, and a Master of Arms Ti gas block. Gonna be light as hell on the front end. Transitions will probably be 'too fast' and take some getting use to.

I haven't done much looking lately but does anyone know off the top of their head what the weights are of a complete poly lower vs a complete aluminum lower?

Not off the top of my head, but if you follow the link in my Sig to the Parts Weights Database, it would be easy to add it up. The NF poly LPK is HELLA light. Most of the heavy steel parts are swapped for poly. Only one you can't do that with is the bolt catch. One company makes a Ti one but it costs over $100. Zing!

There are quite a few parts you don't have to have on an AR.  The LRBHO is 4 of them.

Transparent upper/handguards/stock would be a nice touch for a display/cut away gun. Other than that I can't see a use for it.

I get wanting lightweight, my SHTF rifle is a BCM 14.5" LW middy FSB & B5 handguards. I have a red dot and a compact scope on QD mounts so I can pull them and just use irons. I ran a Limbsaver pad for a while but moved it to another build to save weight. With no optics it feels like an M&P AR15-22.

I swapped the aluminum lower for a poly lower for a while and felt no difference. The scale might have shown a few grams difference but my arms did not. IMHO if you want to shave weight take it from the front of the magwell with a light barrel and FSB or a short light rail.

Like swapping heavy steel wheels on car with lighter alloy wheels, the total weight of the car may have only dropped a few pounds but it FEELS a few hundred pounds lighter due to less rotating mass.

Read up on Magpul's attempt to make a translucent mag, they couldn't make one as durable as colored polymer. Most modern polymers are usually infused with something, like Zytel which is a fiberglass infused resin. Not saying you can't have a strong clear polymer, Lancer does it, but I wouldn't start with a receiver that's 'just plastic'.

had a cavarms lower break at the bolt hold open roll pin when I had it on a table trying to a free a very stuck beta drum from the magwell...

They make ultralight aluminum lowers...

That is true. My UL project actually used a regular aero precision gen 2 lower. It's a SBR so I wanted long term durability and was willing to compromise. I off set this by running the lightest combo of off the shelf upper parts that $ can buy. AL thread pro + faxon pencil 10.5" + master of arms has block + Brigand arms CF carbine hand guard = stupid light.

That would seem to be their one weak spot.  I broke a Sabre Defense LRBHO roll pin boss trying to pound in the LRBHO roll pin w/ a hammer.  GWACS sold me a brand spankin new lower for $35.

http://www.gwacsarmory.com/limited-lifetime-warranty

You have a Mk 1 or Mk 2?