Being 'part of a community' and having a term that defines one's identity are two different conditions. In the former, one's participation in a community is merely another aspect to one's personality or character, which can be all-expansive.

In the latter, one is tied to others who share the identifier. Even if 'rationalist' just means one who subscribes to the importance of instrumental and epistemic rationality in daily life, accepting and embracing that or any identifier can have negatives. The former condition, representing a choice rather than a fact of identity, is absent those negatives while retaining the positive aspects of communal connection.

Exempli gratia:
One is trying to appeal to some high status figure. This high-status figure encounters a 'rationalist', and perceives them as low-status. If One has identified themselves as also being a rationalist, then the high-status person's perception of the 'rationalist' may taint their perception of One.
If One has instead identified themselves as being part of a certain community, to which this 'rationalist' may also claim affiliation, One can claim that while they find the community worthwhile for many pursuits, not all who flock to the community are representative of its worth.

If someone thinks this a losing strategy, please speak up, as it's generally applicable. Notable exceptions to its applicability include claiming oneself as identifiable by their association with a friend group or extended family, as in, "I am James Potter, Marauder," rather than, "I am James Potter, member of the Marauders"; and, "I am a Potter," rather than the simple, "My name is James Potter."