Your're putting it in perspective.
1. Tobirama throws the kunai
2. Tobirama starts moving forward
3. FTGs to Kunai
4. Grabs it and uses the force of him moving+the force of the kunai moving for a greater, faster strike than if he just threw the kunai or tried to run at the opponent.
FTG isn't useless because he got to Madara faster than he would have if he tried to run or use Shunshin.
okay now i get it... well that's a good theory but question remains that that motion forward made tobirama lose as it made him lose balance, also he still shouldn't have held kunai behind him even if he moved forward before teleporting...... but i have to say that's a good theory... what do you think about mine??
edit: one more question.. if tobirama just intended to do what you suggested then wouldn't just ftg lvl 1 have sufficed??if he used ftg lvl 2 that means kunai was flying behind madara and was about to hit him when tobirama intercepted... right? and that seems kinda dumb to intercept a kunai that could have hit... as we have seen shinobi killing others with kunai thrown from a distance....