I can accept that argument in small doses only.
In reality there are times when you need to select your best and most proven players. That means an Ashes series and a World Cup. At other times, selectors should feel more free to experiment.
Test cricket is a unique environment and it takes some getting used to - and all Test players do a fair amount of learning on the job.
Some players will thrive (and outperform their FC records) while others will not make it at all. Performances in first class cricket do not allow you to predict accurately who will and who won't succeed in Test cricket.
Therefore you need to blood players in the knowledge that they may, at least initially, fail, so as to find out who's up to the job, and so as to give them experience of Test cricket in order that they can develop into the best possible Test player who will stay at the top for the longest possible time.
I don't agree that there's ever a time in Test cricket when you should not select your best possible XI. To use a Test series to build for another Test series makes a mockery of the whole concept to me.
Good performances in First-Class are not, by any stretch, a guarantee of a successful Test player - they are merely a suggestion of a possibility. Poor performances in First-Class cricket, however, are an almost certain certificate that a player has next to no chance of success in Test cricket. The likes of Collingwood are in a tiny minority, as I said a while back in this thread. The idea that "performances in first class cricket do not allow you to predict accurately who will and who won't succeed in Test cricket" to me simply lacks logic. The rules of the two games are exactly the same, so therefore if you can do well at the highest level you can and almost certainly will do well at the next level down.
Of course you need to accept that players may initially struggle to come to terms with Test cricket, but that's not, in my book, the remotest of excuses for picking a player when you know he's overwhelmingly likely to struggle initially. You should pick a player only once you believe he has it in him to succeed, then and there, and if he fails it should be a disappointment (though not, of course, one that should make you instantly lose faith).
As for learning on the job - plenty of players learn during their Test careers, but precious few ever learn a great deal
on the field, because as almost any cricketer will tell you, the field is not the place for learning, the field is the place for playing and concentrating only on the next delivery and how to bowl\play it.