Your NT-player can't be injured during the NT-games and you don't lose them for your own games because of international commitments: thse are the reasons why in real life teams get money as a compensation.
Besides, if you train NT-players, the first one to have an advantage by is yourself. Your player gets better, you have the honor of an NT-player and they are posiibly getting sold at higher prices. (So if you bought them before they get into NT = extra $$$)
The choice to keep them and sacrifice a lot of income on their wage is all yours.
I always find it odd that people choose the national team above their own team's succes.
Climbing the BB-mountain. Destination: the top.