No tremulous maps were designed for a grappling hook?? Show me one quake map that was. By your reasoning, how many maps were truly designed for the mass driver zoom? Or jetpack?
The point is: the hook is neither a lame duck weapon addition or a use it all the time requirement or necessity.
Why a grappling hook?
1. As I see it, tremulous moves to slow - slow in the sense that game progression towards the
objectives (killing enemy base) never occurs until SD (for the vast majority of games and servers I frequent). I think a hook would encourage some movement (or at least a few brave attempts) around the many alien camp fests just outside hum base. Are 45 minute games really what tremulous was designed for? Granted, it seems like it just evolved into it rather nicely over time (as server player slots grew). There's nothing wrong with that per say, but I just come from a different perspective of game play.
2. Like it or not, the bunny hop physics exploit on other games does encourage player progression (well beyond the tab tapping and kill whoring fluff fests most who've played trem long enough engage in). So, why not a grapple hook as a parallel enticement?
3. Even without the alien mine addition, a grapple hook by itself would not jeopardize current class or race balance. How many of us have already been through at least 4 of those revisions and modifications for balance? The hook need not be fast either. It could also have some delay in between successive hook attempts. One goon could easily hunt down a fleeing hooker, especially when he sees the slow linear path they will take.
4. The alien mines would not be terribly effective either. When they go off, they would pop 2 to 7 points off hum health, but more importantly, knock down or stun hookers and jetpackers.
5.

6. Fun!
Just think of the possibilities for 6 - a caravan of hookers at S2 takes turns behind the others, swinging into an alien lair for some nade depth charge runs. Or...