Practicing in other styles benefitted my Aikido in a variety of ways. The reverse is also true.
Judo and BJJ training helped me to gain deeper understanding of kuzushi principles, body mechanics principles for kansetsu and shime waza and how to apply the aikido principles of body movement while on the ground (still working on that last one
Training in Chinese and Korean arts (JKD, Wing Chun, Tajiquan, ITF TKD), I got a different appreciation of application of ma ai in my Aikido and was able to explore a variety of aspects that would affect atemi waza principles, especially in the area of targeting, Jin and hand/foot positions, shapes and angles.
Other Jujutsu training such as Daito Ryu and Akayama Ryu reminds me of martial applications of very simple and basic movements found in Aikido that may appear harmless at first.
Non-martial training in Qigong has really helped in the energy and mind focus department, bringing the other principles together quite nicely without confusion so far.
Just my few cents. I'll always be a proponent of cross training. I believe its good to see oneself from within and without to get a comprehensive perspective of things, same goes for MA.
BTW I voted "Very Important".