I'm seeing merit in both sides of everything on the currency subject, and am left completely unsure what the best answer is.
Option 1: Gold is just for social(ish) things ("beer'n'ores," trade, land, castles, hirelings).
Pros: A good set of guidelines/good DM, can make this into a meaningful set of benefits, especially in terms of social interaction within an area of the campaign. There's also some benefits in that wise investments can help pay for adventuring overhead and hirelings can help out with combat and such.
Cons: A lack of guidelines/DM interest could mean that these rewards aren't really present or compelling. A lack of player interest in these things leaves them with no tangible benefit to getting/spending gold.
Option 2: Gold is for "training."
Pros: A powerful "training" system would be easy to implement and provide tangible/desirable rewards for players spending gold. Training also has the potential to differentiate from level-up type benefits.
Cons: This could easily become just a clone of buying magical items, in that, if the rewards are too powerful, players will feel compelled to get them and DMs will have to either make things harder or limit gold acquisition.
Option 3: XP for GP.
Pros: This provides a tangible/desirable reward, with nice possible variations like requiring that you spend gold to get the XP. It also incentivizes players to look for non-combat methods of gold acquisition that may be easier or better than killing everything in sight to level up.
Cons: In conjunction with XP from killing and questing, this becomes a tricky problem to balance, as DMs would have 2 rewards that essentially do the same thing and the speed of advancement depends on the flow of both, which depends on players actions. If this were to be the standalone method of XP acquisition, then some players might be compelled to seek out ways to gain wealth for the benefit, at the expense of doing something less beneficial, but more enjoyable for them (ex. owning a business could net more gold, and consequently XP, than dungeon crawling, but some players just like the crawl above all).
Option 4: Purchasing magical items.
Pros: (Before you throw things, there ARE pros) This system did provide a tangible/desirable reward for getting and spending gold. It was differentiated from the reward for getting XP, and provided an avenue by which players could expand abilities.
Cons: Balancing the game around this system left little/no gold for anything else, tied the hands of DMs in how much gold they had to give out so players could stay competitive with standard monsters, and was a detriment to the grittiness/low fantasy that some preferred in their campaigns.
TL;DR - There're a lot of ins, a lot of outs, a lot of what-have-yous to anything the game does with gold. I don't know which is the right/best one, but maybe DMs should have
all of these options available to them to determine how best to run their particular game.