When you make a perception check, you make two rolls and use the higher result.
This is quite comparable to skill focus: perception.

Math:
Spoiler
Show
1-(1-P)^2 ~ (P+.15)
1-(1-2P+P^2) ~ (P+.15)
2P-P^2 ~ (P+.15)
P-P^2 ~ .15
-P^2+P-.15 ~ 0
They are equal @ [-1 +/- sqrt(1-.6)]/-2
They are equal @ 1/2 +/- sqrt(.1)
They are equal @ 0.816 and 0.184


So if you need a between 5+ to a 17+ to succeed, reroll wins -- if you need a 18+ or better a 4+ or lower, skill focus wins.

Skill focus is, however, a rather weak feat.
You can also use a perception check instead of Arcana to sense the presence of magic.
This is relatively minor fluff.
You can master and preform divination rituals, in addition to three other rituals listed
Also relatively minor.

But between the above, it does accumulate.