I've noticed that when you use secondary fire on grenades, you do an underhand toss instead of overhand. I thought: "Wow, that's cool. 2ndary fire probably rolls more, while primary gets you better range, right?"

Wrong. I did experiments. Secondary fire doesn't travel and less or roll any more then primary fire.

So why's it in there?

I'd say make it so secondary rolls more, so you can roll it down hills, into pipes, under obstacles, etc.

Primary fire travels further and bounces more.

Secondary fire is very useful when you want to place the grenade exactly where you want it, and not have it possibly bounce away (blocking doorways on defense, etc...).  Secondary fire has less range and a lower trajectory as well.

They both have their uses.

I often use secondary fire for my nades if I suspect somebody around a doorway or on the other side of a wall from me, it also works great because many players are in the habit of charging towards a grenade thrower, knowing that the primary fire for nades often causes them to overthrow or at least the bouce often carries the nade clear of them.  If you secondary instead, they will end up running right into your nade rather than away from it.

Got to agree with  
When I have to run backwards and I'm followed by someone, I trow the grenade in secondary fire mode, this way I won't throw over his head, but he will run straight into it.

yea, secondary works good in CTF. if an enemy is going for your flag (either in the bunker, or on the cement platform) it is much easier to get the nade where you want it to go by lobbing it underhand. well, unless you *really* have your timing and aim down, you can probably get it to explode when you throw it and it hits them. i think it is funny when they make a grueling attempt to jump in the bunker and get the flag only to find a pleasantly hot grenade waiting for them.

Secondary i salso good for juuust droppin git off of a balcony when someone is underneath you (Hong Kong Streets). Back to the idea that it doesnt bounce as much.

You'd think that underhand would give you better arc and roll, and overhand would be for precision...

I never ever use secondary grenades. Maybe it's just habit. Whenever i'm on the run, and want to grenade someone, I bounce it off a wall in front of me, and seem to always manage to get them. Dunno how I do it, but I do the most flukey grenade shots, bouncing it off 2 or 3 walls and hitting someone squarely in the face. My most memorable one was during a 1on1 against my clanmate SilverStone, near the blue flag in mp_hk2 facing towards that area, I threw a grenade high, it bounced off the top of the big stone balcony thing, bounced off the right hand wall and landed right in his face, after he jumped off the box. LoL =]

They should have made the secondary fire that you'd pull out the pin out of both grenades and start juggling with them.