It depends on what the objective is
For example if the objective is to own the earth, then slowly replacing our genes with theirs not only achieves the objective, it may give legitimacy to the claim via the possesion is 9/10ths of the law reasoning.
Theres the hybrid vigour scenario
http://www.telegraph.co.uk/science/...h-benefits-of-breeding-with-Neanderthals.html
If any of their galactic contemporarys objected to the invasion, they could claim it was to our "benefit".
Thats if they notice a genetic stealth invasion at all.
Most of that population also think this was to the benefit of the original geneotype.
Better technology, nutrition, housing, medicine. reduved infant mortality rates etc etc
One mans destruction is another mans upgrade.
For an alien species to simply destroy us and take the planet, may well look like an act of hostile destruction, where as a genetic uplift program could be sold as a positive change.
A genetic stealth invasion may simply be a politically smarter way of getting the job done
We tend to look at the possibilitys from a human pov, that may make it impossible to fathom the methods and motivations.
Reading the article above its clear we ARE hybrids
Experts are now convinced that early modern humans and Neanderthals interbred between 50,000 and 100,000 years ago. As a result, between one and four per cent of our DNA comes from the prehistoric creature, according to the research.
That this hybridisation was to our benefit.
To me this raises the potential that some of the genes that make us what we are, may be of a non terrestrial origin.
Humans tend to set a military goal and then go about achieving it as quickly as possible, as others have mentioned that may not be the mindset at play here, a stealth invasion taking many thousands of years may be acceptable to an ET "invader".
It may be the neanderthal is the real "earthling" in this scenario, and that we are the invaders.
Most people assume that modern humans are
A. Native to earth
B. The rightful owners
If modern human hybrids (which is what we are) are the result of a genetic stealth invasion, then its the Nandys who were A and B.
The fact that this idea doesnt even occur to most of the humans on the earth, is in fact the ideal outcome of a stealth invasion, the idea being that no one (terrestrial or otherwise) notices it happening
Where the outright destruction of a native species might provoke the ire of your galactic neighbours, a slow genetic invasion may not.
We see the same thing here on earth, often where a military invasion is not possible, a corporate invasion is.
The cuckoo uses the same tactic, with the surrogate parents completly unaware they are raising and teaching and passing on territory to offspring genetically unrelated to them
Australia is a classic example, when the japanese tried to invade militarily during WW2, it was resisted both intellectually and physically by the most other nations of earth, yet barely 2 centurys prior the white man gene invaded by stealth with barely a shot fired
Where the overt invasion was resisted and objected to, the covert genetic stealth invasion was succesful.
300 years ago the people of australia were all dark skined, walk into any city today, and you will notice the population is for the most part white, the white mans gene is dominant
Most white australians think this was to the original genotypes benefit, better technology better housing, nutrition, medicine, decreased infant mortality rates etc etc.
Where simply destroying us and taking the real estate, may be seen by the greater galactic community as an act of destructive hostile negativity, a genetic stealth invasion could be sold as being to our benefit.
It may simply be wiser politically to do it this way