Identity and groups are basic parts of our species, predating modern society. The strength of these effects, as likely present in our genes and refined by society warrants considerable attention, as their manipulation can be very powerful in shaping and motivating people. Liberals recognise that while movements typically need some symbolism in order to rally people, it suits our values and interests best to consider strong identities dangerous and be skeptical of them.
Strong identification with concepts and principles are definitional for a thinking person, and we do not pass judgement on whether that is in itself harmful. It does not serve those concepts and principles to allow that identification to be prescriptive for those who have taken upon themselves the role of a thinker - for thinkers, these concepts and principles are instead descriptive and should be held more loosely. This degree of independence makes it more difficult for people to be manipulated through these identities - people either don't have to be "a good N" or feel free to have their own idea about what one looks like (or both). This also helps avoid the discreditation of movements by those who do betray their principles - the following should be three separate evaluations: is a movement weakened in theory when one of their leaders does embarasses it by acts that fall outside what it considers acceptable? Is a movement weakened in theory when a series of leaders are led into such behaviours encouraged by conditions the movement creates? (e.g. celibacy in the Catholic church) Is a movement weakened in theory when actions done properly within its purview necessarily lead to bad results?
Movements are best based on principles and honestly evaluated according to those principles. These embodiments should be kept or abandoned without fear of shame from other perspectives, standing or failing on their own merits. People should disown or commit to reform of institutions that do not meet their values as well as they reasonably can be met and avoid fondness for them that would prevent that.
It is thus a liberal value to be skeptical of patriotism - patriotism is a betrayal of whatever principles one holds and through that lens the nation(s) one considers. The principles are ill-served if one identifies with a nation that does not well-realise them, and the nation is ill-served when it is regarded in ways that prevent its improvement (as defined by those values). Liberals consider group identity dangerous and useful - necessary in that societies and movements need some amount of it to survive, but dangerous in that its ability to manipulate can easily cause great harm.