Another place where the linear scaling is a problem is summoning monsters. It's not fun to summon monsters that are far too weak to do anything because they're too low level. That's why in my mod I had MonsterLevelPerUserLevel 1.0 -- this is especially important at higher levels, where the 0.5 factor makes the summoned monsters completely useless, and requires a major investment in the skill for them to be even remotely useful. Instead, I made it so the summoned skeletons/monsters are at the user level - 1. With more flexibility, the monsters could start out at user level - 3 and max out at user level + 2 or some such thing.
It's not fun to gain a level and have your skills be weaker relatively speaking! This is one of the big problems with both linear spells and linear summons. The player expects their skills to keep up with their own level advancement, rather than being forced to invest more because now they're up against higher level monsters.