I have used something similar as a player some time ago: A Sorcerer with access to the Druid list instead of the (sorcerer/)wizard list.
After some sessions, I multiclassed into a sorcerer with 'normal' spell list and was going for a dual progression PrC.
In my opinion, nothing much changed. (at least, at low level. Never got to try out higher level spells)
I don't expect the Bard to gain any spells that would make him more powerfull. Unlike the Druid, he has to pick his spells known at level up, so far less flexible than the Druid is. (unless you plan to change that as well)
On the other hand, the Bard spelllist is customized to his spell progression. Some spells have a lower level than for the Sorcerer/Wizard. This is to ensure he gains access to those at approximately the same level as Wizards do, given his delayed spelllevel progression.
If you give him access to a 'standard' druid list, this will mean you can only get access to 6th lvl spells, and at a later level than the druid can.