It will break protocols that require/use broadcast/multicast on a LAN.
Typically many OS discover network services using multicast/broadcast - so It can break discovery of resources...
printers, filerserver, peoples itune's libraries etc...
Typically clients are chatty - constantly trying to discover neighboring services/clients etc... without blocking this it needs to goto to all AP's that have that essid on the same vlan - and multicast/broacast is typically sent at lowest speeds - since its one packet meant for all/multiple devices on a LAN. so everyone on the bssid needs to acknowledge it. Just use wireshark to see how much bcast/mcast a typical windows/macosx client sends - then extrapolate that for your number of clients... and you get lots of airtime consumed by packets at lowest rate. Clients will still do this so you'll still see this locally on a single AP from clients, but blocking it on the controller keeps bcast/mcast from one AP needed to go to all other AP's - and keeps any wired sources of bcast/mcast from going to all AP's.
I've had it blocked from day-one - and have not run into any negative impacts - in general people want net access to known internet sites etc.... and can care less about discovering and seeing Joe's Vacations pictures shared via itunes... :)
The only negative from enabling it is for places where people expect things to work between wireless devices, like they do at home... ie sharing between iOS devices, DNLA, etc- so might be an issue in a Dorm/Personal environment - look at ClearPass for airgroups as a way to have cake and eat it to