Well, it's rather easy to get +4 to your AGI (spell, muscle-toner, drugs, attribute boost, etc.) at the beginning of your career
Raising your Skill above 6 takes enormous amounts of Karma (just to get to 10 you spend 68 Karma) and training time.
If you really spend all that Karma on one skill, you probably won't mind spending 8 more to get the quality later on too.
Also keep in mind that for every three dice to defense you need to raise your Physical Limit at least by one (which requires raising other attributes).
This is especially important if you are hit by an attack that creates more hits than your physical limit: With Agile Defender you can just reroll misses with edge, with Acrobatic Defender you are forced to add Edge to break the Limit.
The three points more you can get with Gymnastics usually isn't worth the Karma expense