I haven't played the RDF/LT, but I have the Expeditionary and the XM8.

Generally, light tanks are all about taking opportunities that your MBTs are giving you. MBTs are your primary targets, as they're slower than you and don't have an easy way of dealing with you. For the vast majority of the time, your job is to shoot at people without letting them shoot back. This could be because you're flanking an enemy MBT who's paying attention to one of your team's MBTs, you're using your above-average view range to attack MBTs who don't even have a chance to see you, or you're using your excellent acceleration to peek out and fire when the enemy doesn't expect you to, among other things. You're fast enough to get across the map if you need to, and your camo is workable.

Remember that light tanks are the kings of acceleration and gun handling. No class can match your acceleration and you get the unique benefits of light tanks (max accuracy while moving slowly, a speed boost that drastically improves your already insane acceleration for 5 seconds, and 3 smoke grenades every game). It's easy to compare them with the TD line and say "well, the TDs have better camo, equivalent view range, better guns, better top speed, and better DPM", but TDs fill a completely different role because they have their own set of advantages.

Ultimately, that's what it boils down to. If you want to sit back and snipe, play TDs. If you want to flank MBTs, play LTs. If you want to scout and primarily attack squishy targets, play AFVs. That isn't to say that you can't flank with TDs ever (you can, you're just a bit less effective than an LT) or you can't go after squishy targets in an LT (you frequently will) or that a well-played AFV won't completely obliterate some MBTs from time to time (they do so quite effectively), but if one of those roles appeals more to you, play that role.

Note: I am not an expert on TDs or AFVs. I am fully aware that the distinctions between classes tend to blend together with certain vehicles. The above is meant as a general statement of my personal opinion only, and I apologize in advance if it offends anyone.

^{My current stats, if you want proof that I'm not completely terrible at the game.}