It decreases the odds that you'll be hit by about 20%.
If we take a look at the Benchpress Take 4 spreadsheet, we can see how the various levels of AC for a level 7 character do vs the average monster.
|
AC Value |
% Chance of Being Hit |
| High (Blue) |
33 |
5% |
| Medium (Green) |
28 |
30% |
| Low (Orange) |
24 |
50% |
Given that your AC is 21 prior to the application of Spell Shield, the odds of a monster hitting you on an attack of opportunity is roughly 65%. When you use spell shield, this decreases the odds down to a 45% chance instead (40% next level), not really a stellar thing.
Is it Actually Worth it
Not really. Magus utilizes a lot of swift actions already for Spell Recall and their Arcana already, not to mention any swift action spells. While this may be decent to have as a back-up option, if you have other Arcana you're looking to pick up for your build (like wand wielder or spell Blending), you may be better served picking them up instead.
Additionally, as a Magus, it's far better to utilize your attacks for actually making attacks, rather than trying to perform combat maneuvers and needlessly provoking AoOs. The CMD of monsters tends to outscale its AC, not to mention the various immunities vs maneuvers that they receive, and Magus's only source of boosting their CMB is with the Maneuver Mastery Arcana, and only for a particular maneuver. Overall, it's far better to pick up the feats if you're going to try and utilize combat maneuvers, or utilize spells that can make those maneuvers for you.
On Using the Spreadsheet
The thing to remember is that blue numbers on the benchpress sheet are not realistic numbers for a character to meet overall. Casters tend to focus on hitting the numbers for save DCs and hitting touch, while putting their AC and Saves as a secondary focus, whereas martials tend to focus on their EDV (Estimated Damage Value) and their To Hit instead.
As a Gish (a mixture of caster and martial abilities), a Magus focuses on a mix of the two. They don't need to work on their EDV (instead their spells do that for them), but they do need to work on their To Hit. Meanwhile, like a caster, their AC and Saves aren't a primary need, instead upping their DC's and the number of spells they get takes a primary focus, though they do need to keep a closer eye on their defenses than a wizard would. Spells like Bladed Dash become their bread and butter, moving them out of dangerous positions and getting them to the enemy as part of their spell combat full attacks.
