DeMignon: No it's not. See
Trilarion's post above, for a different wording, rather than jumping on the complaint about "not so optional".
The standalone installers are not provided solely for people who do not want to use Galaxy, they're provided for people to have an installable backup copy of their game whether or not they use Galaxy. As for what the defaults should be, that's easy - the defaults for any software should be what the majority of users of that software expect or get the most convenience from. We do not have any rock solid statistical numbers as to what the raw numbers or percentage of GOG users are that prefer to use Galaxy are, but GOG has publicly stated previously that the overwhelming majority of users who bought Witcher 3 on GOG.com installed and launched it via Galaxy. Even without numerical data to back this up it is of absolutely no surprise that the overwhelming majority of PC gamers prefer to use gaming clients, as evidenced by what storefronts the majority of PC gamers money is spent on and what games are the most popular out there. While there are a highly vocal minority of users that decry gaming clients, they are just that - a very small minority whether they like it or care to admit it or not.
Since the majority of gamers and thus the majority of GOGs customers very much do want and use Galaxy, it makes sense that the standalone installers should have support built into them to communicate with Galaxy and register game installations with the client if the customer is using it as a manner of customer convenience, which is what the overwhelming majority of customers are going to want. It has no impact whatsoever on customers that do not use Galaxy, since their games still install no problem and the trivial amount of disk overhead of the goggame-*.info file and other associated files are just negligible.
Other than inadvertent mistakes such as the one someone mentioned above concerning SR4 cropping up, there is no impact on non-Galaxy users, and if and when such mistakes occur as has been pointed out, customer feedback will likely result in the problems being fixed and GOG's internal processes and procedures being updated to hopefully avoid such inadvertent problems in the future.
Punishing all users, the majority of whom do actually use Galaxy according to GOG by not having Galaxy integration in the standalone installers because an incredibly small number of GOG users have Galaxyphobia and OCD about small files wasting negligible space on their hard disks isn't the smart way to go. The number of customers that would hit GOG support with "I installed this game and it doesn't show up in Galaxy" without integration would be multitudes larger than the miniscule number of customers that do not use Galaxy who are worried about a few kilobytes or megabytes of wasted space due to Galaxy integration, or who are paranoid about the installer attempting to invoke Galaxy client if it is installed in order to register with it.
Everyone doesn't need to like it, but GOG needs to run a sustainable business and they'll do that by providing maximum convenience to gamers overall which means doing things that are convenient for the majority while having as little to no impact on others. Sure, there may be the odd case where things went wrong such as the SR4 example above, but that's an example of a flawed quality assurance protocol and not a flaw inherent in what they're trying to do, and the exceptions do not make the rule.
They're generally going to make the defaults for how things work be what the majority wants and expects, not the minority, and when those defaults have no material impact on people, all the complaining and nitpicking in the world isn't going to change anything, nor should it.
