At the risk of repeating myself - isn't that what GP2 is for?
And isn't there a risk that, by throwing in extra F1 racing (either with older cars or less-experienced drivers), you end up slightly devaluing the "main event"?
F1 is the pinnacle (apparently), so by making it exclusive to a mere handful of drivers each season, you maintain it at the apex of all teams'/drivers' ambitions. Simply handing out Sunday-morning mini-races to the hopefuls makes a mockery of that exclusivity (and that's even before we get into the question of financing the extra running).
Sorry to be a wet blanket on this, but if your test/reserve drivers cannot prove themselves in the junior formulae, then why bother having GP3, GP2, F2, WSBR et al? No - there has been a concerted effort to develop a proven ladder for (some) young drivers to attain the goal of F1 - I don't see the point in adding to it, simply to satisfy some people's desire to see more F1 cars on track.
Am I the only person who thinks that cramming more and more races into a season isn't necessarily a good thing? You have to maintain some form of expectation, otherwise you ultimately risk saturating the market to the point of apathy.