That's the risk of mines, if your opponent actually brings scouts to sweep the minefield, they should be rewarded.

On the other hand, well placed mines (ie. on jump in points) can be devastating to an inbound fleet, and obviously provide no exp since they blew up of their own volition.

XP values shouldn't be estimated around how the AI behaves, but how human players do. A human player would focus mines in warp-in points and use them in combination with starbases or tactical buildings, making them are quite dangerous and worthwhile. Otherwise strikecraft should give no XP at all since the AI tends to suicide them into my ships, enabling me to put my fleet at the edge of their gravitywell and farm XP all day.