As You know, many people play with a Harvester Treaty, where both players promise not to attack the harvesters. This includes not guarding the Tiberium fields, or allowing sentries to automatically attack a Harvester that comes near.
Here are 4 reasons why this is not a good idea:
1. It's a hassle. If You have a base located near a Tiberium field that Your opponent's Harvester uses, Your sentries will inevitably attack it, and You will be forced to drop what Your doing to corral them. In the short time I have been playing I have had this happen. Both bases were directly across the same Tiberium field, so both Harvester sets used it. My sentries kept attacking, and I had to keep stopping them. My opponent got so mad that he stopped playing. He accused me of breaking the treaty, eta... when in essence my soldiers and tanks were protecting their base. If You must have a HT, I suggest a modified HT, where automatic sentry fire is excepted.
2. It's bad strategy. Your opponent's income is what allows him to build new Weapons Factory to hurt You. With one less Harvester, that is less income. In order to increase credits stores at the same rate as before, he must spend 1200 to buy a new Harvester, and tie up his weapons factory building it. By destroying a Harvester You force Your opponent to either deal with less income, and therefore less ability to build, or deal with buying a new Harvester and tying up his Weapons Factory. While he is building a new Harvester, he can't build a tank to smash Your infantry.
3. It's not what would happen in war. In the real world, if You have a military operation mining minerals for increasing Your military strength, Your enemy would label stopping that operation as a priority. Especially if the target has no offensive weapons.
4. HT's make the game easier. If I don't have to worry about my Harvester, and source of income, I can focus on other aspects of the game. While this is precisely the reason why many people want HTs, I would say it is a reason to not have them. "No HT" makes the game more challenging.