A lot of people don't realize how important trade can be even WITHOUT GLH. An early trade network with an AI will double up the value of your trade routes. If you have 4-6 cities, this is 4-6 commerce above what you'd get otherwise (in other words 8-12 commerce). Not sounding significant, but this can come at a time in the game where you have little other basic sources of commerce and can easily increase your global by 25% or more with early writing, BEFORE libraries. Now if you have someone with which you can get intercontinental trade w/ sailing, you can push your trade routes to 3pretty soon.
Even without anything special, currency + trade is worth 4if you have OB at all in each city. Across an 8 city empire, you are getting 32 commerce. A city stacked with cottages that have grown a bit will have barely matched that by 1 AD.
Combined with the ability to put up an early great scientist (17 turns of investment running 2 scientists), this makes writing one of the most powerful early research techs and IMO it's gotten too late by a lot of players.
But you can pretty much rely on the AI's having writing way before you, so you get the benefit of trade routes without writing. Actually the economic tech gotten too late by lot of players is sailing, since most of the times you don't have worker turns to waste on roads to the AI.