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(Digel) This is a quantity that depends on position of the spacecraft and time, and so years ago was relegated to the FT2 file. The thinking at the time was that filtering on geomagnetic quantities could be done with GTIs using gtmktime. Now that geomagnetic latitude appears to be of more direct interest to the analysis, having it available in FT1 files would be a convenience. The quantity would not have to be geomagnetic latitude per se - the current implementation derives it from McIlwain L - but either one or the other should be in the FT1 files.
(Jim C) If this is needed for correlation studies with other FT1 quantities, then I suppose there is a reason to have it here, although it would make more sense just to use the merit file directly. However, if this is needed for making selections for ranges of geomagnetic latitude for use in analyses with IRF-based analyses, whether using the ScienceTools or not, then one really should use gtmktime, since the GTIs enable one to compute the livetimes.