"Familiarity breeds contempt" is a maxim thats been around for years. If you travel to other dojos, or attend the larger seminars, you'll see that the level of mediocrity is a local or transient phenomenon that comes and goes -- it's simply the fact that there are people who aren't trying their utmost to do their best at the time
(or who are, but just haven't got it yet). "Mediocrity" is often in the eye of the beholder.
Aikido IS a martial art; how 'martial' is a matter of instruction, intention, and receptivity. Different instructors at different times have different things to impart; if you hang around and travel around long enough, you should begin to see the martial depths to the art. If you only show up on Monday nights to the beginner's class, you get what you pay for.
Here's my belief, after spending over 20 years in the martial arts world: While a few years of training in another martial art might teach you some different fighting fundamentals, unless it is related to Budo, it going to give you new things to think about (and maybe improve your physical fitness) but not otherwise improve your Aikido. If you truly want to "cross train" to improve your Aikido, you'd best pick an art like Karate, Jujitsu, Judo, Iaido -- since they are related arts, applicable things might transfer. (If all else fails, it'll give you a greater appreciation for pain as an instructional tool.)
In my experience, training under different Aikido instructors has been more illuminating to my Aikido development than "cross training" under a different art. Especially training under different Shihans -- Yamada's recent visit to Chicago's MAC was an incredible experience, for example.