Shouldn’t the fi and ffi decompose to /f /dotlessi and /f /f /dotlessi rather than /i? The GlyphData.xml is using the /i.
You can customise it. I do it quite a lot.
http://www.glyphsapp.com/tutorials/roll-your-own-glyph-data
I personally want the dot though, since it’s a nice visual guidance when I modify the terminal of f.
I have my own user xml file too. I just thought it odd to see it set up the way it is, although as you say it is a very good guide for the /f.
Whether you need the dot is dependent on the design. Sometimes you do need a dot or don’t have a dotlessi (I personally don’t ever miss it), and it’s most likely safer to use i as default.