I started in Aikido, but trained in other martial arts (Karate, Tang Soo Do, Judo, Ikebana) to become more rounded and better at doing strong attacks. This is very important to me and I am now introducing more strikes/kicks etc at the beginning of the class so that people can still use these, and so realistic attacks.
I think many martial arts are very similar at higher levels; there are only so many ways to manipulate you and your opponents body - though there have a different emphasis.
I totally agree with mixing martial arts and having mixed martial arts training events. It not only gives you a bigger repertoire, but makes you realise the range of potential attacks and also that a good attacker does not leave himself as open as an uke in aikido may.