While 24h volume is much more easily manipulated it is still more useful imo (if we look at the average of 24h volume).
It is not as meaningful as "Sent transactions" as measured from the block chain instead of only volume reported by exchanges. See my prior post on why.
Transacrions fees vary wildly so I also don't think that's useful. Besides, automated services (like gambling) or even pools which pay out very frequently will push the transaction fees up.
The entire point of using transaction fees is to capture the actual amount of adoption use for a metric that can't be faked because it is a fee that is lost to the miners.
Hash rate is also somewhat useless due to its fluctuating nature.
It is necessary because it indicates the competition that faking the transaction fees by paying to yourself as a miner would cost to achieve. In other words, it measures the security against cheating the transaction fee metric.
Also increased hashrate implies increased security against a 50% attack, assuming the hashrate is well distributed.
That's a pretty big assumption.
It is only necessary that the cost would be implausible of faking the transaction fees by paying them to yourself and significantly impacting the average transaction fees per unit time.