I'm starting to lean towards the thought that ABMs arent worth the planet slots for the silos once your defence gets past a certain size. Unless you happen to be online when the IPM rain comes and have a decent amount of metal and deut to build more ABMs there's not a huge difference between needing say 1000 IPMs to take out your defences and needing 1080-1100.
Am I missing something? 8-10 planet slots seems like a lot to sacrifice for a form of defence that is negligible if you're offline and in my experience not super-effective if you're online because they either won't launch anyway or they're really determined to get your defence down in which case they won't care about a few extra million cost in the IPMs to get the job done...