To the point such designations are worthwhile, the case for Venezuela is fairly clear. There are several senior government and military officials currently under sanction for their role in drugs or arms trafficking. President Hugo Chavez, unfortunately, doesn't take the issue seriously enough to do anything about it. In fact, he tends to promote the people who the US designates as drug traffickers, just to annoy us. Having a known drug trafficker as the top military official in the country is reason enough to say Venezuela is failing in their obligations.
The case for Bolivia is less clear. Yes, Bolivia has some problems with drug production and trafficking, but so do most countries in the hemisphere (including the US). There were some current and former officials arrested this year for ties to drug trafficking, but corruption is also prevalent in many countries (for example, Colombia's former intel chief was sentenced to prison this week for working with drug-trafficking paramilitary groups). As far as I know, there is no public information indicating that the Bolivian government is tolerating organized crime within its ranks. Bolivia is not cooperating with the DEA, kicked out the US ambassador, and allows a certain amount of legal coca growth, but if this is based on those issues, that is a stretch.
I disagree with a number of Bolivian President Evo Morales's government policies, but I don't get the sense that Bolivia is in the same league as Venezuela in terms of failing to stop drug trafficking. Bolivia's problems look remarkably similar to Peru or even Colombia. Its lack of cooperation with the US over political tensions, while difficult, isn't a clear sign that it is failing at stopping drug trafficking. In fact, Bolivia is working closely with Brazil right now because Brazil is the main market and trafficking route for Bolivian cocaine. Bolivia is far from perfect, but I haven't seen evidence to say it's failing any worse than Guatemala, Honduras, El Salvador, or a few others.
I worry that by lumping Bolivia in with Venezuela: 1) This designation loses credibility, looking more like a list of countries that the US doesn't like rather than a list of countries that are failing in their obligations. 2) It diminishes the problem in Venezuela, where the threat of organized crime reaching the top levels of government is very real. Finally, as I've written before regarding other lists, the US government should do a better job establishing clear criteria for this "failed demonstrably" designation. The situation in every country is different, but for international audiences, there should be a clear explanation as to why Venezuela and not Mexico, why Bolivia and not Pakistan. If that explanation cannot be easily summarized, the designation could do more to harm than good for the image of US foreign policy.