Cell hoppers are players with tunnel vision. They're willing to take the cells of other teams they join, without contributing hardly anything back. When the wanted cells are depleted, they move on. It doesn't matter if they stayed long enough to be included in a battle. They leave anyway. They're vision of having or ranking up a certain monster hurts the progress of team members who donate to them and hurts teams if they were inserted into a war. The timers aren't enough. They don't want to participate in wars. That's not why they joined. Team leaders and co-leads don't have any way of knowing if a player has a timer.
In essence, cell jumpers are thieves without fear of punishment. Now, the Library is in progress. More parameters should be implemented to prevent cell thievery.
Suggestions include banning players from joining teams, based upon the number of times they have hopped within a short period of time (2 Weeks) and stats demonstrate equal ratios of the types of cells donated and received. Repeated offenses include incrementally increasing ban time each subsequent time they are banned, with the inability to receive cells for a period of time after the ban is finished and they join a team. This will discourage and punish cell hoppers/jumpers. Limiting their ability to join AND receive cells immediately will put a huge damper on the urge to acquire cells through team hopping.