Mr. Magee, since this is an Aikido forum I assumed that we were talking about Aikido. Of course, if one is a student of MMA, or tennis, or basket weaving then it doesn't really matter. If you don't know what it means to be a serious student (of anything) then maybe that is what you should focus on becoming instead of cross-training.
How do I know my Aikido won't improve unless I try cross-training? I have observed numerous people who came to Aikido from other arts and had trouble because they were trying to apply what they learned elsewhere in Aikido. Kendo practitioners have a difficult time with Aikiken, Karate practicitioners are always trying to block instead of blend, etc.
Personally, I have always wanted to do Iaido; it looks cool and fun to do. But thought if I couldn't practice it seriously then I shouldn't do it. When I have time outside of dojo practice, I practice Aikido with my wife, practice my Aikiken and Jo, surf the web for information about Aikido, read books about Aikido, contemplate Aikido principles, discuss Aikido with my friends, and when I'm doing this or that and try to live my live according to the Way. And, since I'm not anyone's uchideshi I have to squeeze all this between my job, studying Japanese, drinking beer and other life responsibilities. I think if I were to try another Way, it would take away from what little time I have for Aikido.
Have a nice day.
From this I can only think that the only reason you practice aikido is to become good at aikido? Is this a norm here? I do not train in bjj/judo/aikido to be good at them. I train to get into good physical shape, for enjoyment, to develop the ability to win one on one unarmed fights in a ring, to impress chicks, to meet new people and make friends, etc.
I do not think the point of aikido is to be good at aikido. People have a reason why they train. They did not just decide they wanted to be good at aikido. They had a goal, and they decided aikido was a good way to develop that goal. However, some goals will lend themselves better with other types of training, some will lend themselves better with cross training.