SC could allow players to alter the color, damage, range, radius and velocity of projectiles, AOE and enemies affected in a chain lightning spell, potency of restore/fortify/absorb spells, among many other spell attributes, not only combining attributes. SC was never just about learning spells; it was about personalizing spells, making the magic your own. Who would not find some enjoyment in that? And because it could be exploited in Oblivion, does not conclude that must be the case for a spell crafting system in Skyrim.
While I agree that there are benefits to spell crafting (customization being just one of many), I think it boiled down to what was feasible given the new system. Each spell has three uses: a single use, a prolonged use, and a combined use (with the same spell). Given that each spell now has a unique effect for each use (graphically, not statistically), it would be difficult to give the player the power to design graphical effects for each of the three uses. Difficult, but not impossible, because they could have made a list of effects to choose from--just as you describe. I think it came down to time and the developers decided that that time was better spent on other aspects of the game.