The game can become much harder for you in the long run if you don't finish levels with a high enough grade to grab enough experience to level up your squad.
Agreed. Not that I'm having a issue myself, but I do feel the need to rush to complete the mission for this one particular reason. Unlike Fire Emblem, that had a similar system where you earned more Bonus EXP for finishing missions faster, you don't gain experience for killing enemies, so there's no point in killing enemies unless you need to kill them as a requirement for the mission or you need to kill the enemies hold up in the base which you need to capture.
I'll admit that I'm too fond that a majority of the missions are "Capture the base". I wish there was more variety to main storyline missions than "Capture the base", which a majority of the missions are up to Chapter 15. Still, enjoy the mission nonetheless since mission do have variety within them even if the objective is the same.