If you're talking about the roaming Mesprit in D/P/Pl, then no it is not worth chasing.
If you have Black 2, White 2, Omega Ruby, or Alpha Sapphire, you can catch Mesprit without having to chase it down.
However, if you don't own these games and you're looking to work on your Pokedex, then yes it is worth catching since you need it in the Pokedex to complete it. Though if it's easier, you could get a nearby friend to trade one to you if you need to.
Hope I helped!