Other than the mess, algae can coat the roots, starving them of oxygen. Algae also feeds off of the same nutes as the plants, creating competition for the nutes. Algae also seems to raise the PH of the nutrient solution (not fact per se, only in my experience).

If you read an article that said algae is NOT bad, they must have been talking about the algae that forms on top of the rockwool, because that is the only place I know of that it is considered acceptable.

L0rdsOth