This is an upgrade, and also an expansion to the already diminutive ranged side of minecraft. Bows are nice, but can they zoom? They have a set range, and it would be nice to have something better. Take a look at the gold crossbow: it is a completely different from your average bow.
So the point of adding them is to be a total upgrade to the bow? I believe the proper terminology for this situation is "imbalanced."
This is the better alternative to guns.
Not really. A lot of gun suggestions actually attempt to make them have some kind of downside compared to the bow. This makes them a straight-upgrade. Neither type of weapon (gun or crossbow) is needed for the game. Adding new weapons would not make ranged combat in this game any better. In fact, I bet it would be worse. Part of the challenge/fun of the bow is the thrill of monsters rushing towards you while you charge it up. But these would let you roflsnipe them without any risk.
And why not be able to run like hell after you take a potshot?
The slow reload encourages tactics of annoying the enemy, basically sitting real far away, taking a shot, and then hiding while you reload and watch the poor fool you just shot try to figure out where it came from. The bow makes you have to get closer (because long-range targets would require you to basically aim at the sky, which makes accuracy very difficult).