Zipline is by far the most useful and least situational ultimate ability of all. Of course, you have to keep in mind that opponents will use your zipline too, but sometimes you can turn even that into an advantage (e.g., set up a trap, or just use that as free source of information of your opponent's likely movement trajectory).
Bangalore's ult is just glorified zoning tool, caustic's and gibraltar's have small area and are quite situational - you normally use them once per game. Lifeline's ult diminishes in usefulness towards the endgame. Wraith's is mostly just a movespeed boost on high cooldown - a grapple alternative. Mirage's is just an unreliable escape also on very long cooldown - even more situational. Bloodhound's ult is powerful, but also mostly just in-combat ability.
I'm not saying that any of those other ults need buffs or are bad - those legends are balanced through stronger basic and passive abilities - I'm just pointing out that zipline is most definitely NOT weak.
The range on that thing is massive. You can easily reach any height with it, go over mountains, traverse valleys, position over your opponents, sneak over your opponents, get to spots that are otherwise completely inaccessible - possibilities are endless. And best of all, your teammates can use it too, so you don't end up running off alone and it's also often useful out-of-combat too.
Grapple is just a basic mobility tool. You can just use it to gain some extra speed, but you can also use it to quickly gain height, flank over stuff that would take long to climb over and generally take opponents by surprise. Fighting the robot is always a very 3-dimensional fight - you never know where he will pop up. Of course, rugged and variable terrain are his best friends. Flatlands do not favor the robot (but they're pretty much non-existent in King's canyon).
The only reason I myself don't get along too well with the robot is because of his fat fat body - too easy to be hit with so little defensive abilities (but I'm still too inexperienced on the grapple, so I miss a million opportunities to avoid getting shot in the first place).
And yes, those 2 skills are used basically on cooldown by any robot I see (well at least when they have some experience on it, say >100 kills). Having a robot teammate: "Grappling ... grappling ... grappling ... Who want's to go on a zipline? I do! ... grappling ... grappling ...".
