I heard this is how it’s done in War Thunder. It would be much better if Enlisted also did this, sparing people the many frustrations of having an exp card and then joining a match that is already underway, or that was just a quick conquest match, or where your team was made entirely of bots, or where you have high ping, etc. etc. Things which effectively waste an exp card.
Hell, I was using my exp cards from the Even Fight event today (which was great btw). In literally all matches I joined, I had 200+ ping. And in one of them I had high ping and when I joined the enemy team had already taken the 1st point. Extremely frustrating.
Allowing people to choose which matches to use their exp cards, by making exp cards equipable only after a match ends, like in WT, would solve that problem, remove all these frustrations and make exp cards more valuable in general.
Just prevent exp cards from being stacked up (i.e players using 2 or 3 exp cards in the same match) and from being used in custom matches, and I believe there would be no exploits.