The Unknown Sensory Counter
Hand Grenades are worth having if:
a) You're likely to have marines that will respawn a few times.
b) You research them early.
c) You remember to use them.
d) You have someone to cover you when you throw them.
The last of which, oddly enough, fits in nicely with the idea of the marines sticking together to cover each other from stealthy Kharaa.
TheAdj and a few others may have heard of the term "reconaissance by fire", which is a military euphemism for shooting at/shelling/blowing up areas to see if it shakes any enemy loose. This is one of the best uses of handgrenades. If you suspect that there may be a cloaked skulk in a room or a silenced skulk hiding in a vent, toss a handgrenade in. It will drive them back for a moment, allowing you to pass safely/get through the door or it will make them charge into your guys, fully uncloaked by the blast damage.
As a subsection of this, you can add "area denial" to the list of hand grenade applications. In a hive assault under difficult cirumstances, you can have an HMG and a shotgun covering the gate/siege with a constant stream of LMG marines going over the top, hurling grenades along the kharaa's line of attack. Whilst not as good as having a dedicated GLer, it's better than allowing Celerity skulks to maximise their dodging abilities. A hand grenade has done its job if it makes killing skulks any easier. A random grenade has also been known to end a fade's career early on occasion too.
Secondly, it allows you to winkle skulks and gorges out of dead ends, minibases or from behind nodes at no risk to the marines. Whilst the marine "Ginsu Wall of Death" is usually enough to kill a skulk, frequently it will be at the cost of a medpack or two.
As a tertiary use, they can be used for bombing runs on chambers. A trio of marines running into the hive can hurl a volley of grenades at the chambers and then polish off the damaged strutures with LMG/Pistol. If a marine is really
slick, he can time things so that he shoots first, then kills himself with his own grenade, denying the Kharaa his RFK. (This presumes an utterly suicidal hive rush with the exclusive objective of killing upgrade chambers).
Finally, it's an excellent phase clearing device. A couple of marines dying in quick succession but dropping their grenades on the gate is sufficient to kill a wounded fade and certainly any skulks in the vicinity of the gate.
They generate RFK which simply would not have been gained without their usage. So go on, give them a go.