But so in the end network traffic will increase with the fractions of BTC exchanged ... is that not going to slowdown clients/networks ?
Which makes me wonder : do fractions of bitcoins ever get together again after they were split in a transaction ?
Bitcoins don't really have an identity like that. There's just inputs to transactions and outputs of transactions. Every output can be used as a future input.
Alice receives 2 generation transactions worth 100 BTC. She makes a transaction that uses both of them as inputs and sends 100 BTC to Bob. Now Bob has an output transaction of 100 BTC that he can use as the input to another transaction.
Hope that helps.