My response is somewhat like Rhamia's above.
There are a lot of PvP players who carry over the weapons and tactics from PvP into PvE. These are usually the players that promote the use of, and the switching to, various weapon sets. These players generally know what they are doing, but these PvP-based tactics are rarely, if ever, actually needed in PvE. However, they are used to playing that way, and it works for them.
Unfortunately, what you usually end up with, as Rhamia sort of says, are a bunch of less skilled, more PvE oriented players who copy these weapon sets and tactics, without fully understanding why they would use them or what mods etc., are useful. They are, quite frankly, more interested in what looks "leet".
A typical example of the odd-ball thinking of some of these people is the statement that using a spear "keeps you at a distance and out of harm's way". But, if you were to discuss Ranger's bows, these same people would tell you not to use a shortbow (same range as a spear) because it "takes you too close to the action and into harm's way".
After 5+ years of PvE play, I am sure that the actually difference between using one weapon set and another (provided they aren't totally wrong) is basically insignificant. It's more important to feel comfortable with what you are using and to know how the weapon set synergizes with your build.
I've completed BLA with all 11 of my active characters and at least 4 of those were just me, a buddy, and 4 heroes - the rest were PUGs. In no cases was I using anything you would call a "leet" weapon set (or even a weapon not meant for the character).
The closest would be that my Imbagon uses a focus (for the energy - but not management

) rather than a shield. (A Celestial Compass and a Furious Voltaic Spear - I'm not totally against looking leet-ish

)
So, to answer the OP's question - the reasons why spears would be good to have (in PvE) are very minor - it's more about looks than anything else.