Question: I am looking for a single word to describe a group of customers, who merely take advantage of the company's services (they come only for free period and leave afterwards or take the gift and leave again).

So far I came up with a term `Profiteers`, but this is somewhat pejorative, which is not what I want.

Answer: 
I think the word you are looking for is Sampler, from [Merriam-Webster](https://www.merriam-webster.com/dictionary/sampler). It's certainly non-pejorative, and many retailers, such as Amazon, refer to their giveaways as Samples (in their Kindle Books section).

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> Definition of sampler 
>  1
>  :  one that collects, prepares, or examines samples
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I like the term fleeting for this usage.

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> [Fleeting](http://www.dictionary.com/browse/fleeting) - passing swiftly; vanishing quickly; transient; transitory:
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It know it doesn't fit perfectly but it closely describes people who drifted away from my product after the free trial period.

Or you could use Itinerant.

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> [Itinerant](http://www.dictionary.com/browse/itinerant) - a person who travels from place to place, especially for duty or business.
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This could mean that the person was only using your product only for completing an item before moving on with the process (to possibly someone else's product).