the only reason why you'd want to split your coins among multiple keys is if you want more privacy. for example if you don't want the coins you spend be linked to your main stash which you are holding and not touching.
for example have 1 key containing 21
BTC and never touch it but have another containing 1-3
BTC which you use to spend from, trade, ...
as above poster said make sure to not reuse addresses. the reason is not brute force, it can not be brute forced that would mean bitcoin is dead
the reason is when you create a transaction you are revealing your public key and your public key (unlike your bitcoin address in base58) does not have the same hash function behind it.
it is like this:
your private key (if anyone has this, they have your money) ---- ECDSA ----> public key (this is irreversible but we don't know what will happen in 10 years. maybe someone found a flaw in it someday) --- RIPEMD160(SHA256) ----> bitcoin address in base58 (starting with 1 or 3) this is 2 times irreversible hash function which means a lot more security.
and this is one of the reasons they advise against address reuse.