Is collecting miles for Cathay Pacific worth it for semi-frequent long-haul flights?

I think there are actually two parts to the answer to your question.

Firstly - should you join a suitable frequent traveller program? For that, consider how long it'll take to apply, it you qualify, and if there's any fees. Paid for frequent traveller programs are generally not worth it unless you do a lot of travel / stays with that program group, and even then you need to crunch the numbers carefully. Whether you qualify should be clear enough, and how long it takes to apply is normally quite short.

If the answer to those questions is something like sub five minutes, yes I qualify, it's free then joining is a no-brainer. Even if the program has expiry dates on the miles earnt, spend a few minutes to sign up, collect them now, and worry about if you can spend them later on. You can always let them expire, you can't get them back later if you discover your travel changes.

Secondly - which program should you sign up to? Because Cathay Pacific is part of OneWorld, you have quite a bit of choice here. You don't need to sign up with Cathay's Marco Polo, you can pick any of the OW FF programs that operate in Hong Kong (which is most of them)

Based on the amount of flying you say you'll do, and the fact that you're planning to fly cheap economy, you're unlikely to hit any elite tier levels. The first one is OneWorld Ruby, which gives business class checkin (useful at outstations, rarely much of a deal in your carriers home bases), somewhat advanced seat selection, and the odd bit extra with your FF carrier. Because most programs don't give full credit in discounted economy, you probably won't get it, but check carefully as one of them might. The next level up is OneWorld Sapphire, which gets you lounge access, priority boarding, often a bit extra luggage and points, and some other bits (mostly on your home carrier). Bump the class up on all your flights and you might hit this, which'll make flying a lot nicer, but'll cost quite a bit more in tickets to get... (Unless you can manage a mileage run to get it!)

So, assuming you won't get status based on your flying, there are two things to consider. One is how easy it is to earn miles in your travel classes, the other is how easy it is to burn (spend) those miles. If you're in discount economy, look hard at the former, as a lot of programs heavily restrict the miles you earn on discount economy tickets, especially on other carriers. Check the fare classes, as it can make a huge difference! Marco Polo is known to be quite hard to earn miles and status with, but some of the other programs may not give much credit for a cheap CX ticket.

Secondly, what do you want to spend the miles on ideally? Upgrades? Short flights? Long flights? BAEC is quite good for short flights in Europe (reward flight saver cuts taxes dramatically), is good for upgrades (all premium economy tickets are avios upgradable), is great for short redemptions on AA and Qantas, but is bad for long haul economy (taxes and fees are high). I'm not sure about CX. I do know that the FlyerTalk OneWorld forum has a lot of good advice on picking a program, so have a read there!

Oh, and one more thing - can you top up miles with a credit card / hotel stays / car hire etc? That will vary depending on your home country and chosen program. It can make a huge difference if available and generous, check around a bit first if you have time. For example, BAEC Avios are fairly easy to earn in moderate amounts via credit cards in the US, via credit cards and shopping in the UK, but very hard to get elsewhere. CX Marco Polo are almost impossible to get via shopping or credit cards in the UK, but I'm told that in other markets can be done. This likely won't matter if you only plan on getting a small number of miles and may never use the program again, but can make a decent different if you'll use the program a fair bit. Your plans are somewhere in the middle!