fikio: Welcome to FT! I am sure you understand that it is always good to get the miles from your flights or any other activities you'd be undertaking anyway. Moreover, it is also good to concentrate them in one airline (so fly them or their partners as much as possible). I think that much is obvious.

Now we come to whether it makes sense to pursue the miles? To me that means more than simply "not leaving them unclaimed on the table"; instead you'd undertake extra trouble and expense with the purpose of getting more miles.

That to me is worth it if and only if I would (i) fly enough to achieve elite status (25K miles on most carriers), and (ii) fly enough after that to utilize the benefits of that elite status. If either condition is not met, then I would still let the miles accumulate as they do and use them as I can, but I won't undertake any special trouble/expense in pursuing them.

Please note that this decision is carrier by carrier. For example, I actively pursue AA miles and status, but take UA miles only as I happen to fly them. Others may and do choose differently.

Elite status and upgrade policies vary by the carrier, but as a common rule, only flown miles count towards elite status. So you must fly at least 25,000 miles per year to achieve elite status. However, all miles from all sources (bonuses, credit cards, online surveys, etc) can be used to buy award flights or upgrade purchased tickets.

Awards and upgrades are capacity controlled, come with byzantine web of rules that differ for each carrier, and are not always easy to get. However, people do get them; just be prepared for occasional disappointment. With awards it is generally a matter of advance planning. With upgrades, they are harder to get when you have no status but become easier as you progress up the elite ladder.

Good luck!