Combining only increased the quality of the first item by 10% of the quality of the second item, if they were both fully repaired. If they weren't fully repaired, it would only increase the quality by a small amount (if it all...I think it wouldn't increase quality unless it also fully repaired the original item, but I'm not 100% on that.) Repairs were harder back then, so you'd frequently use found parts to repair rather than actually upgrading the parts. If you could craft items, you'd have to craft 3 quality 500 items (couldn't craft 600) to combine for a quality 600.
It wasn't super fast (at least for me) to get high quality weapons. High quality tools were slightly faster, but I've always focused on tools over weapons.