Okay, so being relatively new myself to being a YouTuber, I think I can help you out here.
If you want to do Let's Plays of Pokemon games, a capture card would definitely be worth the investment (it's rather pricey). Alot of people who see gameplay that literally looks like you rigged a camcorder to record the screens won't touch the videos. The capture cards will deliver video as if the viewer is holding the game system themselves.
Now, if you want to go along the route like TamashiiHiroka, who does mostly reviews, Top 10s, and similar, non-gaming-centric content, I would say you wouldn't need one. I think those kinds of videos take footage provided by other videos and just edits it together in the final version. I'm not 100% on this, but often, those kinds of videos won't need one.
Me, I use my 3DS for my Let's Plays, and most of my content is actual gaming videos, but I thought about doing some non-gaming stuff at some point if I can figure out what to make.
It doesn't hurt to own one if you want to make high-quality LP's of Pokemon games, but that's a decision that you really need to be committed to before you drop that amount of money into the devices. (For an idea, to have my first 3DS have the capture device installed cost me $287, and my new 3DSXL I'm getting in January with the capture board already pre-installed, its about $500. If you want to do console Pokemon games, like XD or Coliseum, you would need something like an Elgato capture device, which can run anywhere between $50 to over $100 to buy.
Now my final bit of advice might sound disheartening, but don't be discouraged if this is something you want to do. There are a LOT of PokeTubers out there; and to get noticed nowadays in today's YouTube, you need alot of luck and hard work put in. You can't put your faith in getting well-known enough like TheJWittz or something like that. Alot of these big names have been at this since before YouTube exploded into the massive media outlet it's become. Having said that however, if you know other YouTubers out there who do have any form of a following, perhaps work on collaborations together. Networking is how you will get known. If this is truly a thing you want to pursue, I would do whatever I could to help you out. :)
I hope this helps you out! :)