So I suppose I am thinking of a projecting mage defending himself on the astral, rather than lobbing spells at a materialized spirit that your team has engaged in meatspace. So specifically to address the point of a mage already being able to do damage in the astral: If I'm not mistaken, only direct damage spells work, and you can't use reagents. So you are rolling your base combat spellcasing dice pool, and they are resisting your hits. So they are resisting 4 damage on average from a mage. And not from a safe distance. On the astral, distance means almost nothing, so they will be closing to do melee from you (which is the only way they can attack you I believe). As to having your own spirits, bound spirit tasks are expensive, and you can't always count on having bound spirits, so you have your one summoned spirit. If you are up against more than one spirit, you may be in for it.
So I'm basing this off my chaos mage, with an Int of 5, Will of 7, and Cha of 4, Log 2, with a club for weapon focus. he would be doing 7P plus net hits that the other spirit would be soaking, rather than 4. Right? Seems better to have the option, right? Of course, with a dwarf, will 7, 6 ranks in Astral Combat, specialized with his weapon focus, he's rolling 16 dice to hit, so he shouldn't be missing. Maybe that character is the exception? I also build my mages with Sum to 10 with A in both magic and skills, so again, maybe the exception there as well.
Am I right with the spells for damage in the astral though?