I think there are other good items to consider before aghs on a hero like Lion. If the enemy got lots of magic immunity or has the ability to get the jump on Lion and kill him before he can get a good aghs ulti off then you can probably get more value out of an item like ghost scepter, force staff or euls scepter. Consider mekanism and other items that can help your team and especially your carry to survive a team fight. Maybe you need a bracer to be effective without feeding in the early game. If none of your teammates are building drums then build your bracer into drums. The reasoning for not building costly items like aghs if you're supporting is that you have to be able to help your team survive throughout the mid game. Let's say you're playing against a pushing lineup for example. If you have the support role with Lion and rush aghs at the same time, if the enemy team is smart they will exploit the fact that your team is weak mid game and they will push down all the towers and potentially take barracks before you have your aghs finished on Lion. You have had low impact on the game because you couldn't really fight as well with blank point booster + ogre club on Lion in comparison to if you'd have had force staff or mekanism or a shitload of wards, detection and smokes.
If your Lion player is clearly better than everybody else and is continously getting kills without dying, then aghanims becomes a lot more interesting. You have to be able to farm into the item without loosing too much impact on the game. If Lion is considered a farmer and a carry on your team, then sure, aghanims might be really good in certain situations. But that is true for most aghs upgrades. Maybe you have a competent Magnus ally like you suggested. Maybe your team can hold the enemy carries back from farming BKBs before you have your aghs on Lion.
I also believe that the game is mostly balanced in regards to the pro scene. Maybe aghs Lion works wonders for some players in public matchmaking, but no pros are picking Lion (and if they pick him maybe they never build aghs on him) in competetive. Then icefrog or whoever's balancing the game would be more inclined towards buffing Lion and his aghs upgrade rather than nerfing them. At least I think that's how it works.
Thoughts?