... if you don't have the private key you don't have any Bitcoins (ask the users of Input.io, Bitcoinica, bitfloor, instawallet, etc, etc, etc).
Slight nitpick.
You can have the private key and still not have any bitcoins (when someone else has the private key as well, then it becomes a race to transfer the coins).
That's why I say: If you are not the sole controller of your private keys, you do not have any bitcoins.
Slight nitpic.
You can have bitcoins despite not being the sole controller of your private keys (e.g. certain scenarios involving multi-sig addresses, like 2-of-3 addresses, where it's not necessary to know the other party's private key, and keeping one's own two private keys secret is enough to be safe and to be the only person that can spend coins from that address).
Hmm... I'm not very knowledgeable when it comes to multi-sig.
If you need 2-of-3 to access the coins, and you alone have 2, you can access the coins and you are the sole controller of the keys required to access the coins. If someone else also has the 2 required to access the coins, you are not the sole controller, and it becomes a race (even if intentional) to transfer the coins.
If two people are required to access the coins, each having 1 for a 2-of-3 address, you are not the sole controller, and you alone do not have any bitcoins, you have shared access to some coins. If the other person dies without sharing their key with you, you have nothing.
I stand by what I said. I think you are just making the key more complex, which ultimately changes nothing. Either you alone have access, or you alone do not. What am I missing?
Initially you wrote: "If you are not the sole controller of your private keys, you do not have any bitcoins." which was a response to "having the private key".
What I meant is that it is possible be safe despite not having
all of the private keys (having 2 keys out of 3 of a 2-of-3 address).
Likewise it is possible to not to have the bitcoins despite being the sole controller of (one) private key (in the case of a 2-of-3 address and someone else having the two other keys).
I think it's more about wording/semantics.
and then invest some of the revenue to long term 
