The roulette works by shoving overlevelled players into an instance that one person has queued for. There's nothing more annoying than landing in Castrum and Prae and not seeing the bonus sign, because you're only doing it if you need soldiery and it means someone is farming one of them rather than just doing the roulette, which is silly (the roulette unlocks after Prae, which is immediately after Castrum).
Also while I felt like I was disproportionately getting Castrum too at first, I've found that it's more even. There was one period when I got Castrum every single time for maybe two weeks but then I got a string of prae and it kind of evened out. I can't tell you what it's like now because I haven't done that main scenario roulette consistently since patch 2.3. I ran it once recently for fun with my Warrior and got praetorium.
And yeah since the advent of poetics the longest queue times are going to be for Castrum and Prae because nobody has any reason to do that roulette anymore. They give 100 soldiery but so do all the crystal tower raids and they're actually needed for weekly quests and gear. 100% I would spam WoD over and over for gear and 100 soldiery (and a more likely bonus) than I would the main scenario roulette. I actually have no clue what to do with my soldiery now. The highest soldiery gear you can get is upgradeable to 110 but WoD drops 120 gear for my alts (and isn't remotely difficult) and I've got all the 130 gear for my main and most of the 120 for my alts so... good riddance soldiery. I find myself sitting with 2000 soldiery and having to spend it on trash because I don't like wasting it. This usually results in buying Dubbin from Aelina and selling it on the market board. I have literally no use for soldiery gear anymore.
DRG is still my favourite. I've levelled nearly every class at this point and I enjoy most of them for different reasons. All the melee classes will have a harder time with keeping DPS up compared to ranged in terms of when the boss jumps around the arena (which depends on the boss of course), but the true magic is how strong our attacks are.
I'm using a DPS parser (no one tell SE) now and it tells me exactly how much damage per second each member of the team is doing. Because of this and the new DRG rotation, I'm consistently at the top of the team in terms of DPS. One person beat me last week by 0.2HP/s (another DRG ofc), which just means that he has exactly the same rotation but I got one less hit in owing to having to move away from the boss for a meteor XD. When done right, DRG has the most potential for highest DPS in the game (but you need to use the right rotation and know when to pop jumps which most don't, make no mistake). In second and third - and in my experience tied, are MNK (monk) and NIN (ninja). These only fall very slightly below DRG in terms of raw damage output, but the monk has its stacks to think about and the NIN relies on Dex rather than strength.
All ranged DPS fall slightly below melee but usually end up close to equal because they don't have to worry about positioning the way the melee classes do or dodging close-range AoE. Based on this Bard has got to be one of the easiest, because they don't have to worry about interrupting their casts while dodging ranged AoE. BLM tends to come out on top of raw DPS in the ranged classes though. SMN's have their pets which obviously contribute to their DPS and sometimes put them above BLM's.
So in short, in a spectator match with every class if people aren't optimising their moves you're likely to see Bard, Summoner and Black Mage in the top re: DPS output. BLM will probably win because their regular moves just hit harder. If everyone's playing optimised, the melee will always be top in terms of DPS.