When you purchase a complete rifle does anyone sometimes mix and match the upper and lowers between them. For instance I have a Bushmaster and a Rock River that actually fit tighter when I swap out the uppers.
I see no issue with it personally but I know like anything else there are going to be multiple opinions. I would like to hear the reasons.
All my ARs are frankenguns.
Colt 6920’s are the only guns I keep whole, they are actually the only AR’s I buy whole.
Receiver fit doesn’t really matter.
All of my guns are franken guns to some extent.
Yeah I don’t think I would tear up a colt either.
I know that on an intellectual level it makes no difference but I like a really tight fit between upper and lower. Plus the Rock River has a really nice Trigger.
I just tore up a Colt for my last build. 
I only have Colts but in the past I have had frankenARs by necessity not by choice.
I am plotting on putting together a frankengun, parts gun. Get a lower with the pic you like, and go with top shelf lower parts, and a BCM or LMT upper with or without addons. Stock of choice. Rock and Roll… BTW, I have whole guns,just want something better.
I like my LMTS whole. I do switch out uppers, but with OTHER LMT uppers. I don’t mix brands.
Yeah. I’m kinda nutty about keeping my factory gun original. I only have one factory gun, and it’s a Colt.
Talk about fixing something that isn’t broke 
the only thing original on my m&p15A is the rear sight, handguards, upper and lower recievers, and BCG. I figured out that I like custom features after I bought a complete rifle. 
My dream would be an LMT MRP, but instead I’ll be free-floating with a Daniel Defense Omega 7. does that count? Then a Sig Stoplite. Gosh—the light will be envious of the gun. Maybe. And then a bipod. Not sure on glass yet.
I am sticking with 100% LMT so far. I am working on my second LMT 16" now but, I need a Defender lower. I have a 16" LMT piston MRP upper on backorder and I will need a Defender SOPMOD lower for it.
I think about the only one that I would worry about keeping whole would be Noveske since it has that laser engraved insignia between the upper and lower. It would bug me to see just half of it .
It’s crossed my mind.
I have a Colt 6450 9mm that has a 16" barrel with A1 upper. When I decided I wanted a 9mm SBR I considered just doing the SBR treatment to that gun, but the idea of having to use a non-Colt upper on it, and having to engrave my information onto the lower, made me choose a different path.
A year later I have a “factory” SBR from Anvil Arms with a 9mm RRA upper on it. I haven’t shot it yet because I was waiting to get the correct magazine block from Anvil. I have that now, but haven’t had the time to sit down and change the parts out, and then get it to the range to see how it does.
If I was doing it today I would just SBR the Colt and try to get my hands on a BCM or Noveske 10-10.5" barrel. I think that the quality of those barrels is befitting the quality of the Colt lower.
My method of keeping my 6920 whole: it is completely returnable to stock with unused (never fired) components and no permanent modifications. Improvements include DD Omega 7.0 rail, CTR stock, full MIAD grip plus aluminum Magpul trigger guard, PRI Gas Buster, KNS Gen 2 anti-rotational pins, LaRue-labeled Troy rear BUIS, Aimpoint T-1 on LaRue mount, LaRue F.U.G. vertical grip, & Vortex flash hider.
(Pics to come once the BUIS/vertical grip/KNS arrive next week…)
Only have one, right now, and I’m keeping it whole. A Colt 6920.
Many of my AR’s are frankenguns.
Plan on keeping my “factory” rifles complete.
KAC SR15 E3 IWS.
LMT MRP SBR GP (pending from Grant).
My sort of, but not quite, Frankengun, Colt LE6933HB (via SAW) (pending from a dealer in the Great NW).
My only Frankengun is a Global Tactical Ion Bonded upper and lower (pending LPK)
So I guess I’ve got a real mix of factory, semi-factory, and parts guns.