I think that it depends on what you really want. People who want to "fill holes" usually have some end goal in mind of being ready for anything should the need to defend themselves arise. Other people simply want to do more of what they enjoy. I sing. I enjoy singing choral music, and the majority of my experience is in that type of music. I also enjoy singing rock and roll, blues, and sometimes even country. I don't have to ask if it's okay to sing other styles of music or to get style specific voice coaching. If I want to, I simply do so.
Martial arts suffers from a lot of what I consider to be mostly fortune cookie logic. "You never truly master anything" is a good example. It pours a meaning into the word that is rarely used outside of the martial arts, where mastery is generally used synonymously with proficiency and is considered to exist at different levels rather than being a level above. I differentiate the two words here because it causes two pages of asinine debate over what it means to master something as opposed to simply be proficient.
Another is the idea that if you depart from your one art to pursue something else because you're not getting what you want/need from it, then you're somehow shortchanging yourself. You may be, but that is hardly axiomatic. Part of the problem is that many of us have a lot of our ego wrapped up in the art(s) we practice. Which leads to the notion that 'my art prepares me for everything' mentality that I see and have probably at times participated in.
Generally, I feel that if you're trying to round out your skill set, it is good to find complimentary arts. But if you've been doing taekwondo and suddenly decide you want to be a Talhoffer sword wielding aikidoka who can kick like a mule (from all that taekwondo), then by all means, go and do so. The only person you need to satisfy is yourself, not a bunch of self important keyboard jockeys.