Within a single game, the player will be facing only a few types of monsters. Granted, because of prototyping, the difference between imps in 2 games will be wildly different, but within the same game, they'll all be the same.
So we must consider we only have so many creature types to deal with : imps, giant imps, chess pieces, and maybe a few more.
Going from one planet to the other will grant a much needed change of scenery, but the enemies will remain static.
One of the main reasons to keep playing within ONE session would be adequate leveling of your enemies. Since you get stronger climbing your Echeladder, and get stronger items, your enemies need powerups as well - within balancing of course.
Let's take the canon as example. John, the Greentike (lowes Echeladder rung / Level 1), would be facing Level 1 imps, other enemies are too powerful yet. After some leveling and going through the first gate (ignoring the ogres at the moment, I'm assuming John spent too much time futzing around with his sylladex and too few time to actually leveling), John, Boy Skylark or whatever (Let's pretend this is Level 5) now meets Level 4 to Level 6 imps on the planet, which means some are easier to kill, some a little more challenging. At around Level 10, you meet your first Basilisk. Level ranges can show the player where he's supposed to be (For example, the Underlings and monsters around the Denizen should suitable intimidate a Level 1 player).
Later, John returns to his home. Rose meanwhile has entered, which results in his monsters being a) prototyped and b) levelled up a bit, to reflect the imps won't merely get a graphical upgrade with their prototyping. Player 1 / John can now advance to those stronger enemies, gaining echeladder levels, or abscond to Roses planet, where the imps are still nice and easy - since it would be unfair to subject Rose to whatever John's level is up to now. This nudges you, but doesn't force you to team up on Planet 2.