It's also ideal to spend the UE as you roleplay the activities which would justify the improvements.
One of our players likes to talk about doing the exact opposite, as a JOKE: Saving up 400 UE until they're in a situation where they want to roleplay a Dragonball-style powerup, pausing the confrontation long enough to @assign all that UE to stats, and posing or emoting going Super Saiyan and revealing his "final form".
But that's a joke because it would represent magic, not anything ICly reasonable.
If you choose to roleplay keeping your character mediocre and developmentally frozen, it's your choice, though I hope there are IC reasons, since NOBODY else's character is delayed this way as long as they're roleplaying (obviously, people who are away from the game and aren't earning UE are in a different situtation). And when you finally choose to unfreeze it, I hope there's an IC reason for that as well as for any potential all-at-once investment of weeks' worth of UE.
You earned it, I recommend that you use it. If you can't use it to get anything more out of the game than what you're already getting, then, that's your decision to ignore a feature, it's not our obligation to hide it from you.
Of course, sometimes the solution presents itself. If what you want is to not have this unspent UE, you can certainly avoid earning it at all.