For learning Japanese properly then yeah, katakana is pretty limited in it's usefulness. But in terms of what you'll get out of it versus how much time you need to spend learning it for your average tourist, katana comes out on top. I had been studying Japanese for a year when I went to Japan (part time and somewhat half-heartedly, admittedly) and couldn't read much more than a basic sentence in hiragana and kanji.
But for the most part I didn't need to - what I needed was to be able to tell if I was really buying a carton of milk (miruku!) or some chewing gum (gaamu!). I could have spent that time learning kanji right from the basics, but given my limited vocabulary and grammar knowledge it anything other than say, numbers or the kind of characters that come up in signs would have been utterly useless. If you're aiming to read a newspaper then yeah, katakana makes up a tiny portion of that. But if you just want to get by as a tourist for a few weeks and don't want to dedicate years to studying Japanese beforehand, then I still think katakana is a really good way to start. It's like how phrasebooks are useless for learning a language properly, but nice to use to get by in a country as a tourist.
I think once you get the trick to interpreting katakana it's pretty easy
I find it helps to sound it out loud a few times, even though I probably look like an idiot. It helps if you already have a vague idea of the context - like when my friend was asked me if the bottle he was holding was chewing gum, I couldn't understand 99% of the label but could recognise the katakana for 'gamu', and it
looked like a gum container so it all added up.
Oh! But I do recommend learning the kanji for different types of meat, especially if you can't eat a particular kind for some reason. I didn't know any at the time so we had a lot of mystery meat riceballs, some of which turned out to not contain any meat at all
