In my opinion about this case, it's not so much about the experience (or the lack of it). Vettel is the official third driver at least this year (and AFAIK, has been ever since they parted ways with Villeneuve and "promoted" Kubica to being the second race driver last year). Vettel was the one of the two who not only had tests but the few Fridays early in the season - Timo Glock might have more experience racing in general, but probably didn't have as much mileage in the F1.07.
It's obviously a welcome chance for the team to evaluate Vettel under the circumstances of a "real" race weekend as he might be one of their priority choices for the possible future. I think it's reasonable to assume this will be a one-off incident, though - if there really were indications that Kubica could not be back in the car this year, having a second "more experienced" driver aboard would probably become more important (we've seen just that with McLaren last year, where they favored having De la Rosa in the car instead of trying out Paffett or quickly getting Hamilton in for the last few races).
But for all we know right now, Kubica could be back in the car the next race weekend, so they're probably taking this chance to a) do a test for the possible future and b) have a little extra media attention through this, because now I imagine certain German TV stations could follow them around even more than they already did.
