At a glance the gretchin don't look like a game winner or even a unit that could go toe to toe with anything else in the 40k universe. Their low WS, S, T and I would put most people off choosing them. Grots do have some redeeming qualities. They have a low points cost so you can take lots of them for your waaagh, grots can be taken in mobs of up to 30 which means they can 'tarpit' enemy units and take a lot of punishment. They are a troop choice so can take objectives, being classed as infantry they benefit from the plethora of cover saves now in 40k 5th ed. To cap it all off the models are tiny so will be harder to shoot at.
A Runtherd must be taken for every 10 grots in the unit,
these guys are the ones charged with keeping the grots in line. They are stronger and tougher than there grot brethren. Taking these grants the unit the following special rules
- Furious charge
- Mob rule
the addition of these rules can turn the lowly grots into fearless killing mobs. The Runtherd can be upgraded to have a grot-prod, this grants them poisoned attacks also his squiq hound allows a grot mob to re-roll failed morale tests.
Last but not least grots can be used to clear mines (taking a helluva lot of casualties in return) so the rest of your force can progress un-hindered.
Grots have a lot of draw backs but used correctly they can be a great help to any ork warboss.
Thats it, I hope all you ork generals will think twice before overlooking the lowly grot.