Would this be useless?

Would it be useless learning Hiragana and Katakana if I don't learn Kanji too?
I know it would be very difficult if not impossible to live in Japan without knowing basic kanji, but would I still be able to do other things such as reading a magazine or talking to a japanese person? I started learning Hiragana mostly for fun, and as I don't need it for school or anything I'm not sure I want to learn Kanji too.

If you want to "live" in Japan using the local language, AND if you want to be able to read local magazines, yes, you need to be able to read kanji. Japanese writing use hiragana, katakana, and kanji all combined.

If you just want to be able to speak, you don't "have to" know kanji. But knowing kanji will definitely help to expand your vocabulary when you get to intermediate or advanced level :)

Some beginner's Japanese textbooks come with romanized writing first, but by the time you advance to the next level, they tend to use hiragana, katakana, and kanji combination for the textbooks too.

Reading a magazine? Lol. Afaik, you'll really only be able to read kindergarten-level books without kanji.

Obviously knowing kanji won't affect your speaking abilities, though they will probably give you new vocab.

What are you trying to actually do though? If it's just those things, then you have your answer. Hiragana would also technically be useless though, I would say, unless you want to do Karaoke with furigana. Katakana is handy if you ever plan on going to Japan though.

It's not useless.
If you learn Hiragana and Katakana, you will be able to read some texts and signboards like this page.
http://www3.nhk.or.jp/news/easy/index.html
If you read something like fashion magazines, maybe you can read some keywords.
But maybe you need a friend who teach vogue words like ゆるかわコーデ.
ゆる(yuru); abbreviation of yurui means soft
かわ(kawa); abbreviation of kawaii means cute
コーデ(koode); abbreviation of coordinates

There are many people who live in Japan for years without even knowing kana (hiragana/katakana), or without knowing Japanese for that matter.

That said, if you want to read, including signs and warnings and labels, kana is the least you need to know. From there, however, you can try to learn kanji out of necessity.

For example, train station names are always indicated in both kanji and kana. Things like these are the first kanji that local children become interested in.

You can also read a lot of manga, because many have "furigana" on the kanji, as well as magazines designed for beginners such as the "Hiragana Times".

There are even a lot of Japanese adults who don't know enough kanji, either because they didn't do well in school or lived abroad for so long. Kanji to the Japanese is like difficult spelling to the British.

So you can start from signs, text books, manga, easy essays and so on to learn the kanji that you are interested in or the kanji that you need to read. It's a matter of how often you look up the dictionary.

Again, you can get away without knowing kanji, but Japan is a truly media-oriented country, so you can catch up on a lot of things just by being able to read a lot of kanji.

If you already speak some Japanese, you should also try to hand-write what you want to say. Upon doing so, you will encounter the necessity to look up for the correct kanji to express your opinions. That's how I caught up on a lot of kanji.

Enjoy learning!

Many foreigners learn to recognize many kanji as pictogram evetually. They may not able to pronounce but will get the meaning of them.