even back in 2.0 Bard was still lacking with Auto-attacks. As the developers said they revamped each Job based around their new Mechanic thus by my theory keeping up the Song bar may be required if bards wish to perform at their highest DPS to keep up their 30% damage boost or else they're stuck with 2.0 DPS rate.
It is maybe a revamp measure to make players always play Bard the way it is intended thus avoiding the continue issue we have now where Bard players completely ignore the Songs because they see no person gain from it.
If I have to guess it is mostly likely a build up to the Song's effects. The closer to the end of the Song Bar the more marks obtained thus increasing the effects of each Song but once the Bar reach the end the Song end thus requiring Bards to switch to a new song to keep up their music for party buff and their own damage buff.
It is like classical music where the beginning is weaker then at the end the Song gets their strongest effect to the audience listening to the music for the Grand Finale.
Well that is basically the issue Yoshi-p said the development team was facing when balancing Bard and Machinist. Bard and Machinist are the easiest Job to keep that their 100% damage without any consequences which is why Bard are very popular for players who want easy DPS Job to play as due to havine almost zero risk to their gameplay style.
Certain amount of players voice they wanted Bard to perform at the same Damage rate of Black Mage and Melee Jobs with their DPS while maintaining their Mobility and lack of consequence but Yoshi-P said that would make them too OP so dvelopers gave use what we have in 3.0 where we got higher DPS that we wanted but had to sacrifice mobility. Now people got their Mobility back due to high demand of players wanting to maintain their High DPS while having their high mobility but now it is most likely adjusted so that bard and Machinist still have their high mobility and DPS but they must always maintain their Job mechanic to maintain it.
Machinist still has more risk than Bard in 4.0 as they are getting some close/mid-range skills as part of their weaving skill gameplay style. Ya most players hate the Weaving Focus gameplay style on Machinist but it is how Machinist plays being the Burst Damage Mobile range Job while Bard is the Sustain Condition damage/Support Job.