I guess I tend to regard anything that the player approaches as an optimization problem as a situation with no potential for expressive decisions. Economics is a science, not an art.
Not necessarily. I remember coming across a video that talked about there being two types of decisions in games: "problems" and "choices." The former are the optimization problems you are talking about (what do I need to do to get the highest score, kill these guys and take the least damage, get the best weapon, etc), while the later refer to decisions that do not have an optimal outcome (which character should I romance, what color should my character be, etc).
Right. Optimal outcome: I rescue the Princess. It's my choice whether or not to get the Yoshi coin, because it puts me in danger. It is frequently not optimal at all. I would spend multiple lives getting a Yoshi coin whose only purpose is to give me a life if I get all 5. But, afterwards, it's a point of pride - I got all the Yoshi coins in all the levels! (I actually haven't set out to do this... yet... but have in recent Mario games which have similar ingredients.) This decision is part of what makes me me; this is how I play games. It's not how my roommate plays games. :O
The more I think about it, the more I think of 1P games as a performance art. I was playing VVVVVV time trials at one point and I realized it's all muscle memory and flow - the same things required when I played cello.
Coins should be like 20 to a life, and just be less of them. Maybe even 10. Yoshi coins aren't great because they don't stack - you need all 5 in one level for them to matter at all.
Coins were way cooler back when I was a kid and couldn't figure out their real value. When you get older they are a pass-time and some times monotonous. Coins are great, could be better.
Coins serve multiple purposes, beyond simply giving lives, which wouldn't be possible if their value was increased so. If placed correctly, they can train a player to jump at exactly the right point, optimizing their height. On a mine-cart level, they could hint at where you're going to need to jump. There are rooms full of coins, where the challenge then becomes to collect as many as possible as the screen moves along steadily - it becomes just as urgent as getting to the end of the level, but it's just pure joy in collecting so many freaking coins. It even opens up a new level of potential strategizing and experimentation: should I make long, high jumps or lots of smaller ones?
Though one thing I must say: bananas > coins. Donkey Kong Country 2 made the best use of collectibles I can think of to date.