What makes you think Aikido is not a valid defensive martial art? I'm curious because my daughter wants to study it.
Like I said before I studied Hapkido, which is similar to Aikido and I think it has practical applications.
The Hapkido I've been exposed to is a much harder jujutsu like art that does seem fairly practical. The Aikido that I have been involved with was very impractical and took years to become even moderately proficient. I've been fortunate to get to work with some good Aikido, Aikijujutsu and Jujutsu instructors in my life have had many discussions with them on the topic.
Granted there are many flavors of Aikido out there ranging from the harder Aikijujutsu style of Ueshiba's earlier teachings all the way to the near moving meditation styles of his later teachings. Most of the time when you find an Aikido dojo, it tends to be more towards the latter.
Bottom line, if you are looking to learn an effective means of self defense in a reasonable time frame, Aikido is the last style I would recommend. Look for a school that trains dynamically on unwilling partners, ie near full contact sparring. In my experience those styles are Muay Thai, Judo, Brazilian Jiu jitsu, MMA, Boxing, Kickboxing and the like.