With the current system in place you have 14 slots available for set pieces.
Two 5 set bonuses (on different bars because they are not continuous, but procs) and a monster set take up 12 of these - leaving you 2 slots free to pick:
1. Wading Kilt and a 4 set bonus on both bars
2. An arena weapon
If you look at the Wading Kilt it's going to be worth:
1100 crit (slightly over 5% crit)
10% crit damage
If we are to assume a 50% crit, 80% crit damage setup then:
Without kilt we'd be sitting on a damage modifier of 1.4 (base 1 plus 50% of 80% extra)
With kilt this would be 55% crit 90% crit damage, making it a modifier of 1.495, adding approximately 6.78% damage.
BRP Daggers would add 6% and give us a modifier of 1.4*1.06 or 1.484
If you raise this to 60% crit, 110% crit damage (because of trial buffs) then it becomes base modifier of 1.66:
Kilt: 65% crit for 120% crit damage, modifier becoming 1.78
Daggers 6%*1.66 is 1.7596
So in comparison to a baseline, then the difference between Kilt and Daggers is very small, but you have to factor in the following consideration:
1. Target switching wherein you lose the kilt buff (lasts 20 seconds so not that often)
2. Ramp time of kilt
3. Target switching wherein your blade cloak doesn't proc the 6% and you lose a tick
4. Mostly importantly, the 4 set bonus of 129 spell/weapon damage lost by not being able to 4 set both bars.
To suggest BRP daggers are not competitive is not strictly true - they are very close to kilt in terms of the way the meta runs Nirn/Whorl on different bars.
What I would suggest, however, is that the 6% less damage taken is worth the minute damage lost. Not to mention that using the Daggers adds the perfected bonus of sustain, which is not entirely useless.