Except that hate doesn't reset every time you do the combo. Since just one (1) combo-Halone is the same amount of hate generated as three (3) Halone Spam, you actually have a more secure hold on the mob during the the second low-enmity generation Fast Blade stage of your combo than if you just did four Halone. That is to say, from the 3rd GCD on you will always be better off having done the combo. It's not the end of the world if somebody else pulls the mob off you for one GCD anyway and that's the maximum amount of time a mob could be pulled off of you doing the Halone Combo that could have been retained spamming Halone (since provoke does not interrupt your combo).
After your initial pull where you might do some snap hate generation (usually by blowing FoF, Shield Lobbing as you close, and blowing your offGCD abilities), you should be able to do the Halone combo safely. There are only a few classes that have no ramp-up to hate generation. BLM and BRD can open almost at almost maximum firepower (BLM does need to build up AF, but they have the potential to open with very high burst), but these two classes happen to have an enmity reducing skill that specifically can be used almost right away without being wasted. A WHM can drop massive cures right off the bat, but if somebody does that, they're an idiot.
MNK, DRG, and SMN require several GCD before they hit maximum DPS. A SMN's first large spike of enmity will probably come fully 3-4 GCD into an encounter with fester. A MNK will never have an enmity spike, just high, steady enmity with potential periods of higher, but still smooth enmity and they hit maximum DPS 9+ GCD into an encounter. DRG opens hitting quite hard since their buff/debuff rotation starts with fairly high potency; however, if for some reason the opening pull isn't enough to keep them off you for the 2 GCD that your Halone Combo requires to hit maximum enmity, just one or two flash or uncomboed Halone should keep them off of you and then you should be doing the combo to solidify your hate lead because they're going to hit that fully buffed 300 potency attack like 7+ GCD in and you want to have maximized your lead by then.