As long as all of the items (case, disc, insert, manual, etc.) are original items, it doesn't matter to me if it was pieced together. It's no different than buying a replacement part for your car. It's still the same car.
Now as far as buying a game missing the manual (if it is a game that originally included a manual), under most circumstances I won't do it. It nags me. If it's a high dollar game, missing the manual but for a great price, I definitely consider it. Under those circumstances, you can usually find a manual for sale online.
On the flipside, when digging through boxes at mom & pop shops, if I come across a manual I will usually snag it up, even if it's a game I don't own. Just recently, I found the manuals for both SaGa games for the PS2 as well as instructions and case art inserts for a dozen or more Nintendo DS titles. I currently don't own the 2nd Assassin's Creed DS game, but I found the case, art and manual at the store and snagged it. It's super easy to find the cartridges for sale online and in Gamestop's, but they almost never have the box and manual. I also found the box with art inserts (sadly, no manual) for the 1st Suikoden for the PlayStation.