Not every role-playing game is afflicted with an experience system — classic Traveller and FATE 3.0 are examples of successful games in which player characters’ skills and attributes did not routinely improve through play, while in Call of Cthulhu characters generally got worse (if they survived long enough to change appreciably¹. 00). Among games that are so equipped not all are burdened with long character development path which only a small part is actually fun to play on. With those prolepses accomplished: most RPGs take the lead from D&D. They are designed on the premise that PCs will start out as too feeble and delicate, and have too few reliable abilities, to be fun to play, then pass through a fleeting sweet spot, and then turn into toons².
Is this just an appeal to power-gamers for whom levelling up is better than enjoying the game? Or are there other good reasons for it? Under what circumstances is playing a 1st-level invertebrate before you progress to a fun character justified as a good way to learn the rules in a staged fashion before having to deal with the choice paralysis of a versatile and competent character? Can you do that and then not explode out of the top of human characterisation? Are there other good reasons to bake the zero-to-superhero progression into the basics of the game?
Are there examples of games in which players getting to sacrifice experience points for brief advantages (while having the choice to save them up for game-breaking and fun-spoiling abilities and improvements) worked well? Would any of these break if you capped or cancelled the character improvement?
¹ Which was rare. My few CoC characters either got ripped limb from limb because they were not xenophobic enough to shoot deep ones on first sight, or else went unplayably mad because I could never get them past the point at which looking at things seemed reasonable in character. In one campaign the Keeper introduced a raft of rules tweaks to keep PCs ticking; my character got up to Cthulhu Mythos: 57% (and got his SAN back up to 23!) before I conceded that he had become unplayable.
² For a human, a fall from above 12 metres onto solid ground is survived only with a parachute or by fluke. For a tenth-level D&D character jumping off a fifty-foot cliff may be a cunning way to escape a vampire when you have run out of Protection from Evil and Turn Undead.