Because I did Aikido for about a year and half and it's not what I'd recommend for actual self defense. I think there are things within Aikido that you can learn to suppliment your usual fighting system but as your primary, it's pretty weak.
Like I said, you should go and learn Judo. Judo won't be useful right now but later if you move to a place that actually teaches some sort of combat grappling like Brazilian Jujitsu, the transition will be far smoother.
Also if your place has any boxing or kickboxing, I recommend that as well. Always learn to punch. It usually consists of 70% of the fight or more anyway.
Tae Kwon Do depends. If your place is teaching ITF Tae Kwon Do, go for it! If it's WTF Tae Kwon Do, forget it, unless you just want a very good cardio workout. It is so ridiculously inefficient that you will get uber fit doing it.
In any case, it will depend on your instructor. Some have a "real world" mind set, others do not and have more of a "tradition" mindset. Try to see which one your instructor is. Follow the one with the "real world" mindset. The one with the real world mindset is the one that has a more scientific approach to the martial art. The one obsessed with tradition is the one talking about harmony with nature. Bollocks.