It depends. If you don't fly enough to make status or get their credit card your miles will expire.
If you mainly fly cheap economy fares the number of miles is pretty mininal.
In both cases I would not go thru the effort.

You need to tell us more about your future travel pattern that we can make a better recommendation.

You can credit miles to any Star member airline - there could be better programs than Miles and More, which is relatively good for premium fares (like 3 x Miles for First Class).

Normally you just enter your number when buying the ticket, but you can also register flights afterwards for a certain amoung of time (not sure how long). As long as you don't have status you don't need a card, just your number.