I'll be flying two return trips a year, with a possible extra trip in the rare case of unforeseeable occurrences.
This will be for at least 4 years. The flights are Heathrow - Hong Kong, 5990 Imperial miles each direction, for a total of a little less than 24000 miles per year. Some short flights within East Asia may also be an option, so it may tip above this number a bit.
Are the benefits I would get from a membership great enough to warrant getting one?
It is no hassle at all to collect miles, which you can then redeem (eventually) for almost-free flights. If you will not be flying enough to earn status, you probably don't care what airline you collect into: Cathay or any of their partners will be a fine destination for your miles. The point of an alliance is that you can redeem CP miles for AA flights, or BA miles for CP flights, etc. (If you want to get a credit card that is piped into the FF program to collect non status miles, then there is some benefit to choosing a program based in the country where you live. In addition there are sometimes local promotions that are much easier to play along with if they happen in the country where you live.)
The tricky part is if you think you might achieve status, and choosing the right program to have status in. Most programs offer you some or all of:
All but the last will be offered to you by every partner in the airline's alliance, so unless you think upgrades are super likely or the most important part of your decision, the overarching consideration is likely to be what number of miles you need for status.
Cathay Pacific is in OneWorld. The lowest status level on CP that gets you into lounges is Gold, which is equivalent to Oneworld Sapphire. That level needs anywhere from 50K to 90K status miles (essentially miles flown - there is a multiplier used when you buy expensive fares which probably doesn't apply to you) each year.
So, if you were going to fly about double what you have planned, or if trips within Europe on BA or within North America on AA might add up to 50K miles, you should peruse that table very carefully to try to find a program where you will hit the correct Elite level to get you the "good stuff". But if even the lowest qualification levels are out of reach, then check the redemption tables - I find a 10-20% variation between airlines in Star Alliance and I'm sure it's the same in OW - and choose based on that. Look into buying a lounge pass also.
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!
