I love my baby jogger, bought it for DS1 who was tiny when he came home as he was prem, and used it from newborn with DS2 with no dramas whatsoever. No need at all to buy an extra gadget imho.
I have the baby jogger raincover that is 4 1/2 years old too, never had a problem with it

. TBH it is very rare that we are using the buggy in a huge downpour, it is usually only a passing shower, and I don't think I have used a raincover more than 10 times over the 4 1/2 years. Depends where you live I guess, but down here in Otago I don't need it