on my guild runs our mesmer often runs 2-3 mantras. it’s really up to party comp. our mesmers rarely run signet because we use eles+guardian combo to output strong and long-lasting might stacks, so we just bring mantras to get the pretty strong 4% boost from 30 duelling.
that said colesy’s posts are fairly accurate (even if his words are harsh) and most utilities do serve the party better than mantras. I am just stating how team compositions in organised groups can change the gameplay of the various classes, along with specific tactics (e.g. we use nullfield on golem boss in COE but not feedback because we don’t need the reflection).
Arguably, Disenchanter is better, given you have more options for running Mantras (or other utilities) as it has a better boon-rip uptime than Null Field. Unless you’re killing the boss before you can get a second Disenchanter up, then Null Field is potentially worse.
Additionally, the 4% damage boost from having Mantra’s charged (up to +16% for having 4 Mantra’s ready) doesn’t actually affect Mesmers that much. This trait falls into the cesspit of Mesmers with +X% damage boost. It’s only effective when using something like a Greatsword or a Shatter build (Greatsword specifically for it’s high direct damage output. You can also include a normal Sword, and even a Scepter, to some degree, but they don’t offer as much damage on as low Cooldowns.) This is because +X% boosts do not affect Illusions or conditions. A Mesmer’s only source of direct damage is through auto-attacks, shatters, Mantra of Pain, reflects, traits (such as deal damage when interrupting), and other weapon skills (GS235, Sword24, Scepter23, Torch4, Pistol5, Staff5, Spear5, Trident25).
The actual direct damage access that Mesmers have is very, very low compared to other professions and most [non-shatter] builds don’t utilize them to the point of making +X% boosts worthwhile.
Also, in regards to most of what colesy is saying:
He tends to run in dedicated groups which have different playstyles than running PUG, which is what the forum question threads tend to cater towards. Another reason why is that a full damage build will not hit 15kWarden without the assistance of other party members, additionally, the damage output you, as a Mesmer, are going to drop on the Slave Driver is going to be very negligible considering the rest of the party is geared, traited, and playing towards high DPS. (Keeping in mind that the party is at 25 stacks of might + Fury and every banner dropped while the Slave Driver has 25 Vulnerability [or near, in most cases] for colesy’s expected dps output.) The damage a Mesmer will do in this fight (and the rest of the dungeon, for that matter) is not considered at all. A Mesmer’s duty in CoF P1 is to provide Reflects, provide a Portal at Boulders, provide boons (via SoI), and provide Time Warp. The additional DPS of a Mesmer will not make or break the speedrun group (Warriors will be able to hit 2-3x a Warden in about the same time it takes to cast the Warden. In addition to that, they also have access to more than one skill that deals high DPS like that. A typical "full speed CoF P1 party will have GS+A/A for 100b, Evisc, Dual Strike, Whirling Axe, Cyclone Axe.)
There is really no point in talking about super speedrun dedicated groups on the forums, though, as they are a very niche type of play for very specific and narrow purposes.