Couple of thoughts on the subject:
1. Since it is a demo that wasn't meant to feature Romans, we might not be able to see all eventual formations. (For instance I did not seen any proper Roman manipular line extension or traditional legion formation tool for the commander.) From what I've read Triarii were the last vestige of the phalanx in Roman armies of the period. That doesn't mean they actually were able to form a true phalanx anymore. They might not even have trained to do so.
2. Forming a phalanx as Triarii probably wouldn't work, because of the spears they had. That does not mean they wouldn't work well as a spear formation--they should have good mobility, just not phalanx capabilities.
3. From what I've seen so far the Triarii are just a bit weaker than the Carthaginian Phalanx man-for-man (same unit size), so they should not be crippled by this (and remember they have better flexibility.)
4. Triarii don't have pila.
5. Triarii should still have decent effect against cav because they are spear armed. (Rock paper scissors.)
6. Historically in this period the Triarii were not used unless things were going very badly--usually a fiasco of the 1st order. So it makes sense that they would be spear armed. The spear is good for defense, and for stopping cavalry. When things go badly, the enemy cav is likely to be unleashed (or already has.) Triarii were the emergency reserve.
7. Rome had a real weakness historically dealing with cavalry.