Well, what I'm trying to get at is that by the time you buy a 10 man Warrior squad, and add a Lord with Res Orb, adding a scythe on top is a relative point increase of about 5%. Which is my awkward way of talking about point cost for the Scythe.
So assuming that we run into a 5 man, standard Termie unit, that unit is probably comprised of a Sarge with Power Sword, and 4 guys with Power Fists.
While I don't normally go for a reliance on averages, I'll take a look at the "probable" outcomes of Termies charging the squad outlined above. For those that are curious, the points values for the two units is nearly equal.
So we assume that somehow, neither unit has taken casualties before the Termies reach combat.
The Sarge issues a challenge, to try to take out the Lord before the Lord can start whomping Marines. So with his bonus attack, the Sarge has a 42% chance to inflict at least one wound.
Usually, the Lord survives to attack. The Lord's attacks have a 48% chance to inflict a wound. With these numbers in mind, we have 3 possible outcomes. Sarge wins, Lord wins, Both live. However, thanks to Ever Living and a Res Orb, the odds of a Sarge wins get cut in half... because the Lord can stand back up. So on the turn that Termies charge, the Sarge wins 21%, the Lord wins 28%, and they dance around 51% of the time. 79% of the time, you win or move onto another round, assuming you pass your break test...
Moving onto the next turn, the Sarge's offence goes down notably. He only has a 31% chance to take the Lord out before he can attack. The Lord still has a 48% chance to down the Sarge, and the new outcomes become Sarge wins 15%, Lord wins 33%, and Both Live 52%.
In all instances, the Lord is more likely to kill the Sarge than the Sarge is to eliminate the Lord, thanks to the Res Orb. From there, he's free to attack the normal Termies...
The 10 Warriors have a 35% chance to take out one or more Termies before they attack, so we'll assume they fail. 4 Termies will typically crush 5 Warriors on the charge, and then 2 stand back up. In all likelihood, you've lost combat by 4, anyhow, so your odds of passing the initial break test are mediocre at best.
But if you get through that, the Termies are down to crushing 3 guys a turn, and we'll say two stand back up, and maybe the Lord offs the Sarge this round. A break test at -2 isn't so bad, so we likely continue.
We'll guess that the 6 remaining Warriors get lucky and down a Termie... they've thrown enough crap at the wall, something stuck. If the Lord gets lucky, he downs a guy, but we'll assume he fails. 3 Termies squeeze 3 guys, 1 stands up... combat at -2, Necrons stick.
4 Warriors whiff... Lord gets one Termie. 2 Termies off 2 Warriors, one stands up.
3 Warriors whiff, Lord whiffs, 2 Termies kill a couple more guys, and you probably fail because you've taken break tests on 5 consecutive assault phases... and get wiped.
That's the way I'd see a reasonably typical combat going, anyhow, presuming you pass the morale check on the first round... so you've got to be a little lucky there. With only 2 guys left, they should be fairly easy pickings for something else to wipe them out. I've also assumed that full strength squads run into each other. The Termies start to lose damage for each wound lost. The Necrons only lose damage output if the Lord bites it. If you can whittle the termies down by even 2 models before Assault starts, the 10 man Warrior squad has a good chance of winning, since they're unlikely to take a -4 break test.
I've also assumed that no Wraiths are available for support.
For the points, if you already have an Over / Lord, I think a Scythe is cheap insurance for when things go bad.