why.. ? or are we now reduced to the angels on a head of a pin style arguments much favoured by one list member? Of course if the mounted could dismount, they might be of use...
??
Two possibilities come to mind. One is to defend the TF with Ps (S). The other is to have the TF's intended defenders as part of a delayed command.
I accept that both situations are far from ideal. But rather than angels on heads of pins, I'm looking at ways that players can use rules in ways their opponents are unlikely to expect: if you claim the deployment modifier opponents are going to expect a nearly entirely mounted army running all over the place, not one that plonks down a length of TF.
The Ps will defend at factor 5, and get a +1 if they lose against foot, but repulse if they lose; but they're cheap troops to hold a piece of otherwise open terrain with while you go and Do Stuff elsewhere. As for the delayed command, you might be able to use a road to help the delayed troops move up to the fortifications. Or your opponent might be tempted to race to the fortifications in the hope of getting their before your delayed troops, and not pay enough attention to what the rest of your army is doing...
For what it's worth, the army I was thinking of applying this to is the Late Achaemenid Persian, which has the distinction (with NKE, Early Byzantine and Aztec) of being an army which can field an inert regular C-in-C with three regular sub-generals. Having an inert C-in-C means you have to move second, and that in turn means it would be good to maximise your chance of deploying second. But with an Aggression of 1, you're more likely to be defender, which means you're more likely to deploy first. But if you take an army which qualifies for the deployment modifier, what can you do to stop heavy infantry armies from grinding forwards and driving you off the table? Put down some TF and force your opponent to fight his way over them.