When TRO 3055 came out, I hated the Dart. I wanted every machine to make perfect game sense, and be efficient, and it did neither. The only time a Small Pulse ever had an advantage over a Medium Laser was at range 1, and even then it did less damage. The Dart variant with mediums made a lot more sense. With the TW change to infantry rules, I looked at it again and I kind of like it now.
The Dart's job is to zoom across the battlefield getting a +4 TMM, stop in heavy woods or some other cover, and annihilate a couple squads of infantry. Then it runs away again. It really should be a cleanup machine, one that stays away from real combat and just skirts the edges of the engagement area looking for targets of opportunity. Oh, you've got a squad of dudes sitting in a jeep, acting as lookouts for enemy activity? The Dart runs up and murders them. Yeah they'll get a distress call out, but that doesn't really matter because the bulk of your forces is far away.
The Dart can also operate in your enemy's backfield or hunt damaged units. I wouldn't want to fight a MadCat with one under any circumstances, but it might be okay if the Clan mech already had an arm blown off, with two engine hits and a stripped side torso. It's limping back to the dropship 30 km away with a damaged hip, and suddenly this little bastard shows up and starts pecking at you while moving for +4 and hiding in an arc where you don't have weapons anymore. If it loses initiative, it moves where there's no LOS. If he wins, he runs back over and hassles you some more. He's fast enough to get away from anything except a Dasher, so I can see how it's useful. Very situational, and totally dependent on the user to find a good role for it, but it's not the worst mech out there.