but this can cause confusion, if i want to differentiate the income on my wallet, if i want that, for example all my mining earning go to one address all my signature earning go to another, and i don't like to create a new wallet each time
You can do whatever you like. You don't have to do things the best way if you don't want to. By re-using addresses you are reducing the anonymity and security of yourself as well as many others, but there is no authority to stop you.
There are other ways to differentiate and group payments from the same source. Just two examples are using separate wallets, or using coin-control features of some wallets.
but what he say it's true, when you exceed the 100 pre-generated addresses
No. He didn't say "when you exceed the 100 pre-generated addresses". He said:
that each time you generate a new key, your last backup will not have this key.
That implies that you have to back up after EVERY transaction, not after 100 transactions. It also doesn't take into account the fact that you can increase the size of your key pool.
Additionally he said:
I think backing up your wallet.dat file every week is enough.
Which is completely useless advice.
If someone has less than 100 transactions per year, then they are wasting a lot of time and effort backing up EVERY WEEK. If they have more than 100 transactions per day, then telling them to back up every week could result in losing a significant amount of bitcoins.
I suggest that you both take your signature spam elsewhere. We really don't need all this horrible advice being spread around.