I believe that Twitter waiting on including advertising is a good thing. Not because there is anything wrong with ads, rather they are making strides to not become the cesspool of off-topic advertising we see over at Facebook, Digg among other Web 2.0 sites.
Honestly, I believe Twitter taking their time with monetizing their efforts will afford them opportunities to find “pro-level” services that they can offer that perhaps will mean we will see a Twitter devoid of advertising completely.
What remains unclear for the time being is whether or not we will see Twitter turn into a bubble in the meantime. Despite claims that Facebook is profitable, I would say that Twitter is walking a very fine line in which a massive mistake could lead to its downfall financially.