There are a number of ways a selection can be "justified" and very rare is it that a player is selected without any justification at all.
Strauss's selection is justified based on his previous record in what was a great start to a test career, and his good first class record to back it up. It is not justified based on his current form - but you can say that about virtually every player at one stage or another during their careers - their selection is justified in other ways.
Whether he should be in the side or not is a totally different matter though. Firstly, you have to ask yourself whether he himself has done enough to be dropped. Well, given the relatively short nature of his career, and the fairly long run of failures, you'd have to say yes. But it doesn't end here - and this is a mistake many cricket followers make. As many of us only see the top level of cricket played, we tend to focus on the players in it - ie. is is he doing well enough v isn't he doing well enough. However, this is only the first step in "dropping" a player - you must also ask yourself if there are better candidates who would do a better job who aren't currently being selected. As Richard has stated, there aren't any openers banging down the door in County Cricket. One could select a middle order batsman and make Michael Vaughan open - but is Strauss really that much of a lost cause that you'd weaken another player and throw a rookie in for him, when it could all turn around for him quickly? I think not, really. Strauss's form is bad - very bad - bordering on somewhat funny at times - but he deserves at least until the time that there's another opener pushing for selection, IMO.