if you tell your users that you use btc-e when it is up, you must use it when it is up.  and you must not use it when the servers are down.  otherwise you are committing fraud.  it does not matter if their servers are intermittently up and down, you must keep update your index in realtime, which you are not doing. for example, btc-e is down right now, but you are factoring it into your index, which is fraud