Apart from which remember that 0.9.8 is really 0.9.08 as far as the numbering scheme Kovid uses represents. Based on 0.7.x and 0.8.x there could easily be another 40-50 releases (i.e. another year) before Kovid feels the desire to make it a 1.0 officially. I would suspect he has certain items on his todo list (such as the refactored database backend) that he wants done before he considers it 1.0.
This is not aimed at calibre/Sigil BTW but personally I've never really agreed with projects using 0.x numbering schemes which are actively being developed over many years. It puts some users off installing them (because they would rather wait for the 1.0 version). If you were googling for a tool to do something, and saw one at version 0.5 and another at version 1.2 which would you start with?
For that reason my plugins all start at 1.0 - the only time I make an exception to this is a genuine alpha release that I put in the development forum. However that is just my opinion, and equally every developer running their project can decide how they like to use their numbering schemes. I know user_none has stated certain features he wants to see in Sigil before it gets called "1.0" and I'm guessing Kovid feels similar.
Both are open source and both aren't going away any time soon so just don't worry about it...