The Dart Monkey is one of the best towers to start a game with, outside of Heroes due to their low cost and effective damage and uprade potential for the earlier rounds.
In most situations, using the Free Dart Monkey is the better choice than the Free Glue Gunner as the additional popping power is always a useful bonus.
The Dart Monkey is an effective tower to deal with the first few Camo bloons in a game before more permanent anti-Camo measures can be installed, as it's cheap camo detection upgrade can be easily built into and also flows nicely into it's third Path.
While the Dart Monkey tends to fall off in later rounds, it's not without use. There are several effective strategies involving the Dart, including Super Monkey FanClubs and the elusive Apex Plasma Master.
3 0-0-0 Dart Monkies is a fine way to start most maps, especially on harder modes and if you aren't using a cheap Hero.