Even though plastic is readily accepted, many smaller shops will not take plastic for small purchases. In the US you can pay $1.50 in Starbucks with plastic but that will not happen in Argentina; Cabs do not accept credit card; if you go to any of the markets (San Telmo for example) they will not take plastic.
If you also buy in a smaller/privately owned shop, you can negotiate a better price if you pay in cash vs. plastic.
I find that I use much more cash in Argentina than I do in France or even in Italy (a cash-loving society).
I also rent an apartment when I am in Argentina and they require payment in cash (US$) on arrival.
You get the best rate around with no fees at the Banco de la Nacion in the airport; it is usually better than any of the commercial banks in the city; You can also exchange pesos back into US$ when you leave the country without any fees so I usually prefer to have more in cash.
Enjoy BAires. It is a great place.