For small poor countries it's a smart move to adopt bitcoin as currency, because they have nothing to lose. For this reason the volatility is an acceptable risk to be taken. On the other hand, if more countries like El Salvador adopt bitcoin as well, the volatility tends to decrease along the time, as millions of people will be consistently entering this ecosystem to deal with bitcoin in a daily basis.
I hope these news coming from El Salvador aren't just propaganda and that the legislative house accepts the president's proposal to make bitcoin official there. The world will be watching this crypto experience closely.
El Salvador has been using the US dollar long before Venezuela and if this happens Bitcoin will also be a legal tender together with the US dollar. With that being said I don't think that their citizens will be inclined in choosing a volatile currency like Bitcoin as their main mode of payment as compared to just using USD for their daily transactions. The recognition of Bitcoin will just give them an option but they will most likely see it as an investment rather than a currency. This will also depend if how the businesses will react to this kind of change whether or not they are prepared to adopt Bitcoin as a mode of payment.
Well, for using dollar as official currency they are already much better than most america's countries. Since they have the dollar stability probably bitcoin will be mainly used as investment, like you said. As I see, the idea of adopting bitcoin by El Salvador's president is due to the fact the country has considerable percentage of its income coming from foreigner countries (probably people native from El Salvador who live in USA and other countries sending money monthly to their relatives) being heavily taxed by banks. They saw in bitcoin a potential solution for this issue, so people can send money from any place of the world paying less fees and avoiding the eager banks' taxes.