The thing with Ogrovs is that you can get a ton of them and nuke all planet structures before the player realizes what happened. To make assault nanites useful, you would need like 10 of those capital ships. Plus they are on cooldown and use a ton of antimatter, so you can only have like 4 of them up at a time for each capital ship.
1. Assault nanites are free and use no fleet supply.
2. They only cost 40 antimatter. You'll restore that before they expire.
Yeah, they won't have the assault power of massed Ogrovs, but they'll be a heck of a lot more useful than combat nanites at clearing out planets and they're great for clearing out the defenses of unimportant planets by themselves. I agree you should put more points on combat drones but it can be useful to get a point in assualt nanites for this situation.