Jseš and jsi

In colloquial Czech the non-standard form jseš is often heard instead of jsi, but the interesting thing is that it only crops up when the verb být (to be) is used as a full verb, not as an auxiliary. We would thus have Jseš unavenej? (Are you tired?), but Šel jsi na poštu? (Did you go to the post office?) Jseš doesn't seem possible as an auxiliary. There is a similar situation in Serbo-Croatian, which uses the full or the abbreviated forms depending on a few factors of the verb to be as a linking verb , but only the abbreviated forms as the auxiliary for the past: jesam and sam, jesi and si, jeste and je, jesmo and smo, jeste and ste, jesu and su.