I've already seen the Dervish Vs. Trebuchet comparison and as a heavy Dervish user, need to point something out here.
The two designs, while similar, are intended to fulfill two different roles.
The Trebuchet is intended to support Medium and Heavy line units which don't rely on mobility as much as firepower and armor. The fluff of the Trebuchet even hints at this, stating that it was intended to act as a partner to the Centurion, a well known medium line unit. Because of this, the Trebuchet focuses more on heavier firepower at the expense of mobility.
The Dervish, on the other hand, is intended to support Medium and Light units which rely more on mobility than on armor and firepower and are more likely to encounter different types of units, such as armor or conventional infantry forces. Reflecting this, the Dervish's design engineers made the decision to focus more on mobility and weapon systems which could counter non-'Mech forces while still being useful against 'Mechs, such as the SRM-2s. As it's already been pointed out, the ability to load a ton of standard SRM ammo as well as a ton of Inferno ammo was not an unintended consequence.
As for usage, use the Trebuchet like a miniature Archer, following behind the main lines and providing LRM support where required. Finding a good firing position and providing overwatch is perfectly acceptable, but mixing it up with front line forces is not recommended unless absolutely necessary. You may have three medium lasers as secondary weapons, but that's not going to work very well against a heavily armored and armed line unit.
The Dervish, on the other hand, is best used by also following behind the main force but due to their nature, you're not as likely to have a "line" to support like the Trebuchet would. Your mobility is your best asset, keeping you with your allied units and keeping you from being isolated and destroyed. In the event that you're attacked by short range units trying to silence the LRM support, and chances are you will be, use your medium lasers and SRMS to beat them back. Now would be the time to use those inferno rounds if you have them. Unlike the Trebuchet, who is likely to face line units with thick armor and heavy weapons, the Dervish is more likely to face light and mobile medium units with thinner armor and short range weapons. This doesn't mean you can stand and slug it out, as even these weapons will kill a Dervish eventually, but it does mean you have a few turns to get them off of you.
Boiled down, this is what you're looking at here. Dervish is Cavalry while Trebuchet is Line.
Hope that helps.
