I do tend to agree with this. We all have different interests, and sometimes our circles don't overlap with our partners' in the Venn Diagram of our relationships. Ignoring the common response of "but why do you have to have RELATIONSHIPS to do that when I do that stuff with my friends," (which is something that, as a person pretty firmly planted in the monogamous side of the mono/poly spectrum, I just don't personally Grok), there's another issue with this idea that doesn't usually get discussed.
When you limit interests to only your partners who share that common interest, then you're only painting a partial picture of yourself to other partner(s).
Regardless of whether or not we "click" on, say, comic books, I want to get to know my partner... and yes, that includes being introduced to that interest. I may go to one con, only to never go again, but at least I've been introduced to it. I can see your giddy interest in it - can see you act like a little kid when you meet your favorite artist - and I can appreciate that, even if I don't appreciate the interest itself, or ever choose to go to another con. You've shared that with me.
If you have a partner who also appreciates comics, and you only go to cons with them because that's the person you share that interest with? It prevents me from ever seeing that giddy, little kid side of you... and I think that's a shame.
I don't want to see only some facets of my partner; I want to get to know them. All of them. At least, as much as I can.
I understand things like Cons involve resources like vacation time, money, etc., and those by their very nature are limited. I understand that, because of this, the most "bang for your buck" will be to bring the partner who actually enjoys these activities. However, you run the risk of becoming compartmentalized to your different partners - becoming different people, depending on your audience. And I don't know... if you prefer your relationships to be a big network of people you can diversify around, then clearly that's okay, but if you're in an entwined partnership? Especially with a monogamous person who doesn't have it in their nature to want to diversify? It seems to miss the point. I suppose Polyamory gives you both options.
At any rate, I know people can't be everything to each other, but I personally like the chance to share something that I never would have tried if I hadn't been introduced to it.
Ready for the "but" now?
There's always a "but"...
The flipside to this is when people try too hard to BE ALL THE THINGS. Having something shared with you doesn't mean you're obligated to actually enjoy it, and it certainly shouldn't pressure you to either do so, or fake it when you don't; it just gives you greater insight into what makes your partner tick. This is the point behind "you can't be everything to your partner." Enjoy the sharing. Let your horizons get expanded a bit. And if you find that it's not your bag (baby), that's okay. Send your partner off with a smooch and a "have a great time" and know that they're going to be a giddy little kid on the other side.
And if you're feeling shut out? Ask. Just be ready to hear your partner gush about their favorite comic book artist for a while.
Disclaimer: The examples herein do not represent any person, living or dead. That is to say... I *like* comic cons.