This is a question for those who are native speakers of Lithuanian or have a very good proficiency in Lithuanian as a second language. As for your feeling, are the definite or pronominalized adjectives rare, in the contemporary standard language? Both written and spoken. Or they are as frequent as the simple adjectives, or maybe even more frequent than the simple ones?
A native speaker here.
They are definitely not rare, one can treat them as commonplace. And not just adjectives, but also pronominalized participles and pronouns. But they are also not as frequently used as the simple versions, since they do serve a different and definite purpose, namely to denote a permanent and distinctive feature by either emphasizing (e.g. "trečioji vidurinė mokykla" - the third secondary school), defining (e.g. "gražioji lietuvaitė" - the beautiful Lithuanian girl) or singling out (e.g. " naujieji kaimynai" - the new neighbors).