There will definitely be a disadvantage to having no airstairs, but if easy keep on top of their handling agents it shouldn't prove to be too much of a problem.
Who says they won't have airstairs. It is an option if they want it, the first BA (ex BCAL) 320's have airstairs, costs a few quid (minus easyjet discount of course) and weighs about 300kg. Worth it if you use them a lot, if you mainly park on jet ways then it's not worth hauling the extra weight around for 25 years.