what is the purpose of not having your commerce slider..

[tipsy09]

to the highest it can be? why put it at 50%? mind you...im starting on settler and moving my way up...

i have 8 cities...1090AD...Washington is producing 82 beakers

 

[Elandal]

Commonly you keep it at the highest that you can. Sometimes you keep it so high that your net earnings go to negative (eg. -3 gold per turn) if you have some amount of gold stashed away (eg. have 100 gold in treasure). And sometimes you drop the slider to 0% for a while, eg. to gather gold for unit upgrades.

 

[Mulholland]

Some games you don't have a choice. After a particularly long offensive you could have 8 new cities and a bloated Army forcing your reseach rate way down.
It can be usefull too before you get a new Military tech to bring it down stockpile some gold and as soon as you research the tech, upgrade your experienced macemen into rifles and go on a killing spree.

 

[JoeBlade]

Yes, stockpiling gold for upgrades after being the first to discover a specific military tech is often a worthwhile approach. Stockpiling can also be very useful for the purchase of a key wonder once Univ. Suffrage is enabled.

Otherwise I'd agree though, there's no point in not running the max. possible research percentage. Well, none that I've discovered thusfar.