First and foremost is simplicity for the recipient: if wallet X contains the desired number of bitcoins, the sender paid. Unambiguously. Otherwise it's difficult to tell who sent what, since you might be sending any number of pieces of coins that all make up "your" transaction, which is complex / nigh-impossible to solve without other ways of verifying (such as including a message in the transaction). This is made even more complex if you receive many transactions from many people with a single address.
Second is anonymity. If you reuse "your" wallet and it's ever connected to you, so is every transaction out of it, forever. If you value the anonymity side of Bitcoin, it's very important, but not sufficient. If you don't care about anonymity, the only other downside is that someone who gets your private key can wait until your wallet is bigger before stealing the bitcoins. If you constantly change addresses, the old private key is essentially worthless as soon as you make any transaction. Honestly that's pretty unlikely, and if they have your private key it's game over anyway, they can steal it all at any time they want.
First and foremost is simplicity for the recipient: if wallet X contains the desired number of bitcoins, the sender paid. Unambiguously. Otherwise it's difficult to tell who sent what, since you might be sending any number of pieces of coins that all make up "your" transaction, which is complex / nigh-impossible to solve without other ways of verifying (such as including a message in the transaction). This is made even more complex if you receive many transactions from many people with a single address.
Second is anonymity. If you reuse "your" wallet and it's ever connected to you, so is every transaction out of it, forever. If you value the anonymity side of Bitcoin, it's very important, but not sufficient. If you don't care about anonymity, the only other downside is that someone who gets your private key can wait until your wallet is bigger before stealing the bitcoins. If you constantly change addresses, the old private key is essentially worthless as soon as you make any transaction. Honestly that's pretty unlikely, and if they have your private key it's game over anyway, they can steal it all at any time they want.