The only time when Unown catching was important (to the least extent) was in G/S/C, you had the Unowndex, and there was the capability to print them off with a GB printer, so you either had the optional goal to collect all of them or just let it slide. FR/LG and R/S/E has no Unowndex, nor the GBA never had a printer peripheral of it's own like it's predecessors did. But still, you have that optional goal to collect them all, despite the lack of those features. Plus, you have to catch at least 1 of any kind to add it's entry to the Pokedex. But collecting all of them is merely an additional challenge some could consider accepting.
Plus, you could use Unown to write stuff, like secret messages, as they are in the form of letters.