As to the OP's topic, depending on your riding style and terrain foot retention is not mandatory with brakes, but since your speed for a particular gear is based entirely on your cadence not having the straps/cages will definitely put a cap on your safe max speed. I've been riding all winter with a front brake and bare platforms, and I have found that I need something so that I don't have to slow down so much on downhills and flats.

pedaling on level ground is analgous to dribbling a basket ball. most of the work is used for the downstroke and a toeclip or clipless pedal in this instance is helpful for keeping you foot in place, thats it. however climbing, especially standing efforts are when they come into play. the instep (top part of foot) needs to be strapped down in some way, otherwise you end up sliding forward and loading your knee and tibia muscles in weird ways. in fact you shouldn't be doing much standing riding anyway, but thats another story....

You shouldn't be doing standing riding without foot retention or in general?

Riding fixed gear in even moderately hilly terrain there's not much choice, when you run out of grunt uphill it's either stand or walk.
The opposite effect going down, sit and spin it out w/ a touch of the brake(s) when things get to blender speed.
Proper foot retention for each and in between.

-Bandera

in general, seated is always best. you are best in control of the bike and using the least amount of energy. that standing sprint with all the big guys rocking the bike back and fourth at the end of the race is always the most exciting but the rest of the race was in the saddle conserving juice.......buttttttt, obviously this isn't the case in the real world especially riding one gear bikes. really we are trying to protect the knees. and knees dont like to be overload or twisted. some kind of retention system also helps keep the foot in place and leg track straight among other things. im not too on board with fixed gear myself, just single speed but i can see its allure. im40 and creaky from other dumb $h!t as a kid so i like to coast too!

I agree with the efficiency of pedaling seated, but only if you are in the right gear. When you are in too tall a gear (as often happens on a single speed, fixed or otherwise) then, I find you place less strain on your knees by standing up because you are not bending them as far. I am in my fifties so I have to mind my knees a bit more now or they will complain.

^^^no, you are definitely right. you have to stand at some point on one gear bikes. unless you are senor iron legs and can sit and chug it out up big hills, well then im not that guy. so like everybody else i stand up but im definitely using toeclips or clipless pedals. i havent figured out the magic gear ratio. im still rolling around on the stock 42 x 16 my paddy wagon came with and that seems to get me by on most stuff. any hill over 8 percent and forget it, i gotta walk.

No shame in walking. The bigbarstardhill on my commute has an 8% stretch and a 10% stretch. Walking is sort of essential somewhere along it.
For me, standing up is just a precursor to the end as it spikes my heart rate and I'm stuffed in not time. It's a technique you use if you're nearly at the top, lower down, you're better of walking.

And yes, I'm in my late fifties and there's far more of me than there should be.

There is an old axiom in cycle touring: "Walking is your lowest gear"

And since this is a Foot Retention thread, and in the spirit of full disclosure, I almost always wear foot retention. SPD because they allow me to walk around (and up hills )