Rationalising the utility of swap in Samsung Galaxy Ace S5830
According to this discussion on MoDaCo, the RAM throughput of an Acer Liquid is 1.3 GB/s.  Even if the Ace's RAM is slower (unlikely) it's got to be way faster than the Class 10 SD card speed (10 MB/s write).

So yes, your theoretical basis is invalid given that Class 10 is the highest rating.  Even UHS-II cards with a "theoretical" max speed of over 300 MB/s are slow compared to RAM, and few phones support SDXC cards anyways.