常識的に考えて

屋台でカードが使えるかと同じ程度の問題でしょう。
少額だと手数料も馬鹿にならないので、敢えてカード加盟店になる理由は見あたらないのでは。

そもそもカードを使おうと思ったことがないし、使っているのを見たことがないのですが、
この際なので確認すると。

英国とか米国人が質問するサイトで見てみましょう。

//www.gomadrid.com/travelspain-faq.html

General Questions about Spain

1) Are travellers' cheques accepted ? What about credit cards ?
Generally speaking travellers cheques will not be accepted in Spain.
While it will be possible to change traveller's cheques in banks
and possibly some of the larger hotels, the best idea is to bring
your credit cards, an ATM card or exchange money before you leave
for Spain. Credit cards are universally accepted in hotels, restaurants,
gas/petrol stations, etc. A bank debit card will normally give you better
exchange rates than a credit card, but please check with your bank first.
Cash and small change will be needed for bars and daily necessities.

最後にちゃんと書いてありますよね。

それにバル（イタリア語と違い、スペイン語はBARは[a]であり、[a:]とは発音しま
せん）で100ユーロは使わないでしょう。毎晩通っても数百ユーロでしょう。

ついでに、
3) I don't know any Spanish - will I be able get by with just speaking English ?

You will need to practice a bit of Spanish in smaller bars, shops
and cafeterias. Most Spaniards will make the effort to understand
if you at least make the effort to try to say the odd word in their language.

物価は、
Food & Drink Budget:

* Coffee and a pastry or toast for breakfast is under 2€ (add 1.50€ for freshly squeezed orange juice)
* A baguette and a drink for lunch will cost about 5€.
* A three course 'menu del dia' can cost anything from 6€ to 15€. A la Carta food is more expensive, but you can still eat well for under 20€.
* Spain is hot - factor into your budget the need to buy water and stop for a cool drink occasionally. A 1.5L bottle of water from a street vendor or convenience store is 1.50€ (60c in a supermarket, though it will rarely be cold). Potato chips and a chocolate bar will cost you 2€ for the two.
* Total Food Budget Per Day: 15-40€ (if you eat lunch from a supermarket you can eat even cheaper).

まずは、バルに入って地元民がしていることを良く観察してみて下さい。
社交場でもあるので、スペイン語は必須です。