I don't know that I'd say it is a big word but rather that it overly complicates things. If I talk about time I say that it took a week or I will see you for a week. It seems kind of silly to switch from talking about week(s) to fortnight. It lacks continuity.
and see for me at least as an aussie it is completely "continuous" to say will you stay a week or a fortnight? everyone knows 2 weeks = fortnight and fortnight is quicker to say.
Also you get your government payments "fortnightly"
when you use the online Australian government tax calculator you can select that you get paid "weekly", "fortnightly" or "monthly".
If you rent your house the amount you pay is a "per week" amount, but again you can negotiate to pay it "weekly", "fortnightly", or "monthly". - actually most repayments to banks I think you can elect one of the three options above.
until e-hell i didn't realise "fortnight" was such an unusual word. it is used so often here.
also now I have written it so many times it looks