Sure, you can raise a skill to 7 or even 8 before you take JoaT, and that would save you 4 karma. Meanwhile, the 5 runs it takes you to earn that 30 karma to raise the primary skill from 6 to 8 you might otherwise learn 30 skills at level 1 or *just* 10 skills at level 2.
IMHO, the utility of having an extra 20-30 skill points spread thin is way more than having 2 skill points in a single skill. If you ask me, that advantage is totally worth the 4 or 8 or even 10 karma you might pay later in "interest".