It may not be an ideal situation, but team has many ways to analyze performance, like telemetry. An average F1 fan might find it difficult to tell whether both are really good drivers, but the team will know. To find the next real ace? And in order to find him they need to go through many drivers, so can't keep the same ones for long. Also giving opportunities (even if short ones) to many drivers helps with marketing too, because in that way more countries and markets get covered by their F1 programme. So the strategy is that reasonably good ones are given a couple of years in F1. Those, who are vettelesque, will get more. One problem though is that STR drivers find it difficult to negotiate with other teams. I am not really aware of their freedom in this regard and whether any team would have been interested in, say, Alguersuari. So they are sort of living in their own dimension, cut away from the rest of the F1 world.