Yes it is bad because a lot of bias review sited are present to ruin the average. Im not getting into this too much but when people have a certain logic that if a game isn't above xx score that it isn't worth playing, a system that takes the average of a bunch of site that all have different criteria for their grading process symply cannot work.
Example some sites see a 6.5 as an average but very good game while others say that 7.5 is normal and anything lower is sub par. You can't combine these scores and expect an accurate measure of quality.
The best thing to do is pick s few review sites that you agree with and read what they have to say and get your impressions that way. Unfortunately most people are blind to this and just want a convenient number due to sheer laziness when it comes to reading.