Provided you are out of Cast System.
This is exactly the thing : you should try to play towards Caste System.
There are two things you mention from the Markets.
a) The extra specialist slots, allowing to run 4 specs from Library + Market.
The problem with this, compared with Caste, is the lack of control. Caste allows you to run either 4 scientists or 4 merchants. And the certainty allows you to plan accordingly (do you want to bulb or trade mission ?).
b) Percentages. 2 merchants yield 6 gold, the market's bonus bumps that to 7,5 gold and it is finally rounded down to 7.
So, effectively, the gold bonus from the market, if it is solely used to run merchants is +1Gpt.
Percentages can be good but you need to look at the base value. In a city doing 50 commerce per turn... if you have use for your gold slider... a market could make sense.
But, early game, the situations are very rare where we have such strong commerce cities. Early game, 150H is also a huge investment (Caste is free, remember).
If we're looking at percentages, the Bank is also a much better investment than the Market.
For these reasons, Market spam is often reserved to Space Races and heavy cottage economies. In those situations, you can make use both of the happy bonus and the multiplier.
If you compare with other buildings, the multiplier from the market and the grocer are among the most hammer intensive in the game, only surpassed by the university.
This is why people look for alternative ways to produce gold.
Investing hammers into science multipliers and having other sources for gold is a way to save on infrastructure.
If you have Library + Market, your overall multiplier for commerce (remember : base value is >>> than multiplier) is 25%. And you invested 240H for it.
Alternatively, you could spend 150H into Library + Monastery and 90H towards an alternative source of gold. Your science multiplier would be 35%.
If your capital has an Academy, it benefits from an extra +50% towards science and this is an extra incentive to use the gold slider as little as possible.
Early on, it is much easier to stack science multipliers (Banks and Espionage buildings come late, right ?), and this is why people try to use their commerce production and multiplication, as much as possible, towards science.
1) It's easier
2) It actually pulls you ahead ; conspicuously enough, gold production does nothing (except for keeping us afloat).
With a small empire they could compensate upkeep cost so you will run 100% research slider.
Yes, you could even out-tech a larger Empire... for a time. The problem is that you will eventually be out-teched and out-produced by Empires that control more land.
Remember : percentages have no meaning if we don't look at the base values. 100% of 100 is 100, 40% of 1000 is 400. (And if you run 40% of 1000, Banks can be great in your super science cities.)
I did that exact experiment here :
https://forums.civfanatics.com/threads/stumbling-back-at-deity.673881/page-10#post-16186517
The tech situation is great. However, the problem is that I have exactly 0 production and no way to invade a neighbour. So, I will soon reach a plateau with no way to win the game. Maximizing commerce means cottages but those are very poor tiles to build an army.
Without land and the means to produce units, the situation will keep becoming more grim and grim.
You can compare that play to that of Pangaea's and will notice how he ends up in a much (much) better situation.