Dragon Lance + Sniper, and other similar itemization questions

I've played with a few Snipers in recent games, and the trend I've noticed is for them to pick up an early Dragon Lance. I understand the item is a mid-game item that can be disassembled later for Butterfly/BKB or so, but why do people build an attack range buffing item on a hero that already has an insane range due to Take Aim? It seems to me like he has a few other glaring weaknesses that the early money could be spent addressing.

This topic made me consider another point, which was this: in general, is it better to accentuate your strengths and leave potential weaknesses or is it preferable to be a better all-around hero? It would be crazy to see a Riki, Broodmother, or Bounty Hunter build a Shadow Blade, right? But building a non-7th slot Moon Shard isn't out of the question on Night Stalker, who already gets great night vision and attack speed from his Hunter in the Night.

Or let's look at three heroes, each with a skill that can get them close to the enemy: Puck, Magnus, and Spectre. Blink Dagger is core on the first two, but isn't picked up at all on Spectre. Clearly there is a reason it works for the first two, but maybe I'm just not getting it. Thanks in advance for the insight, as it will probably help me itemize for heroes in the future.

TL;DR: Is it better to build items that cover up your hero's weaknesses or bolster your strengths?