What's up with the notion that, "low magic = useless magic", idea?
In most low magic settings I know, (George R.R. Martins Westeros for example), magic is rare, but extremely powerful, when Melisandre for example resists the poison or uses some spells to summon shadows to do her bidding. In the Kingkiller chronicles, magic would is, in game terms, ridicolously overpowered. I mean the main character beats like 20 bandits and a "demon" by using his magic creatively.
If you make a "magic sucks" setting, don't confuse it with low-magic. If each common magic spell was it's own skill, and casting it at higher magnitudes caused you to take a 10% hit to it. Having bladesharp as that skill would simply mean you have a skill were you can sometimes add half of it to another skill. That's pretty silly.
However to be more constructive than just saying nerfing magic is a bad idea I present one of my own ideas:
Keep the 4 systems of magic but format them like follows:
Common Magic: Keep as is, Common Magic ain't particularly powerful without the Divine Magic Amplify.
Divine Magic: Magnitude is now 1/20% pact skill, Instead of each deity only offering a select few spells each deity adds or deducts a certain amount on each spell of your chance to get them. Want to get Bearform from the god of hugging animals? Fine that adds +20% to your attempt to it, if you want to get it from the god of hitting people with sticks, that deducts 40% from your attempt. This reduces the need for the pact skill, increasing it is not as important, and as a result only fanatic followers will be able to do a variety of stuff from their gods, while the initiates are restricted to their gods favorite spells. In addition to this you could scrap the entire Initiate, Acolyte, Runepriest/runelord, and have it just be Initiate spells are regained at your full pact, Acolyte skills at half your pact, and runepriest spells at a quarter of your pact.
All in all, divine magic gets more miracle like as the powerful stuff gets cast less. Amplify could be skipped although I am uncertain it really is that powerful or whether it just makes common magic worth it.
Sorcery: Intensity (the per 10% of sorcery skill thing) gets tied into magnitude.
In one smooth stroke all sorcery spells now cost 1 more magic point to cast and take 1 more CA, meaning sorcery buffing in combat takes a lot longer.
Spirit Magic: Only chaos spirits can discorporate unwilling people (per the normal rules).
While spirit magic buffs are good, the worst part was that you can run around with a wolf spirit, who gives you 2 wolf companions, and in combat it discorporates one of your enemies. All in all you are fighting like 3 people while everybody else is fighting like one person on an intensity 2 spirit.
Discorporating people is just too good, so I suggest limiting it to a special kind of spirit.