Why not use `revert()` in the refund code?

[aleybovich]

@sandeeppanda92

In the following code:

function () external payable {
      uint256 tokensThatWillBeMintedAfterPurchase = msg.value.mul(rate);
      if ((stage == CrowdsaleStage.PreICO) && (token.totalSupply() + tokensThatWillBeMintedAfterPurchase > totalTokensForSaleDuringPreICO)) {
        msg.sender.transfer(msg.value); // Refund them
        EthRefunded("PreICO Limit Hit");
        return;
      }

      buyTokens(msg.sender);

      if (stage == CrowdsaleStage.PreICO) {
          totalWeiRaisedDuringPreICO = totalWeiRaisedDuringPreICO.add(msg.value);
      }
  }

why are you sending ETH back like msg.sender.transfer(msg.value); (which will spend the contract's own gas) instead of simply calling revert() - that will revert the whole transaction, refunding the user their ETH and gas leftovers if any.

Disclaimer: I am only learning Solidity and Ehereum smart contracts, so there is a good possibility that I am wrong. In that case, please explain me why - that will be very helpful for me.