translating through GT over writing your own thing in first place just gives you some more trouble.
By and large that's how it is. Speaking for myself here.
If I want to do the translation properly, it takes me at worst the same amount of time to type the text myself while I go over it in my head with a certain view of how it should look, how the sentences should flow - which is mostly done intuitively. Reading the machine translation, reading the source, translating the source, comparing the two translations and pondering whether the AI-supplied version is unacceptable, could be improved, or is just written in a different style to yours, is more busy work, mentally taxing, and breaks the flow. Even if there's little to correct in the end. And if you try to cut corners - just skim it for obvious errors - you end up with an unreliable franken-text that you shouldn't be charging anyone for (but I know some people do - there are hacks in every profession).
In short - more work or lower quality. Sometimes both.
Sometimes you get clients who try to pull a 'I don't want to pay a full rate so just correct this GT-spewed text, lol' on you. Or better yet, supply you with an English text that was obviously machine-translated from, say, Chinese, and ask you to produce a sensible result in a third language. We are not fond of those people.
But machine translations can be integrated with the workflow more sensibly than just using GT wholesale. Professionals these days tend to use industry-specific software to manage their work (Trados, MemQ), which can use machine translators to provide suggestions as you type or if you're stuck on some passage. The way I see it, these are more like all those fancy smart tools in Photoshop or some such, that an artist may use to speed up or improve their work.