It really depends on maps and play style. If I build a deck that has 15 creatures that always wins on the maps it's designed for, it doesn't need more creatures.
Using your same logic, you can only play one creature per turn (unless you're using a support deck or packing Intrusion or Replace). So why is an opening hand with three creatures ideal? You can only use one on your first turn.
I find having multiple spells in my hand is quite useful, as many of my spell-heavy decks are designed around flexible, multiturn combos or using certain cards as deterrents.
A proficient spell user (as I'm sure you are) knows that an unused spell card is far from worthless. Holding certain cards in your hand, rather than using them, often has a far greater effect than if you were to use all your spells upon drawing them. Keeping a Revival, for example, ensures that your opponent has to deal with it before they use Suppression to get rid of any spells you really care about. Holding onto Haste ensures that your opponent won't cast Mesozoic Song on you with the hopes of using Intrusion + Saint next turn. Holding onto Senility means your opponent won't use Quicksand, or use Relief to move that Kelpie directly into your path. Carrying a card like Subsidence will make your opponent think twice about investing the G to level anything up to 5. Keeping multiple spells in your hand (as well as making sure your opponent knows you have certain spells in your deck, even as you discard them) is really all about adding flexibility, and knowing what to keep and what to discard is its own metagame.
I'd say many of my decks are around 50 percent spells, in part because I find the typical creature/item-based ST-vs.-HP game painfully boring. But that's a bit misleading, as some of those spells are cards like Wind of Hope, Prophecy and Find, which increase churn and mean I'll draw a creature more often than the percentages would indicate. Additionally, cards like Goblin's Lair and Spartoi are counted as spells, but they put creatures into play.