Why can/can't souls in purgatory pray?

I know two sources on this Catholic issue. St. Thomas Aquinas says no. *ST II-II, Q83 A11 ad 3*: >Those who are in Purgatory though they are above us on account of their impeccability, yet they are below us as to the pains which they suffer: and in this respect they are not in a condition to pray, but rather in a condition that requires us to pray for them. [Edit: I find that Aquinas specifies that they aren't in a condition to pray effectively because having died they are beyond increasing their merit, which means they can't effectively do meritorious acts in the world like intercede. No, I don't get it either. An explanation of this would be welcome!] But the Catechism (CCC 958) shows a different view : > [T]he Church ... ‘because it is a holy and a wholesome thought to pray for the dead that they may be loosed from their sins’ she offers her suffrages for them.”500 Our prayer for them is capable not only of helping them, but also of making **their intercession** for us effective. > > 500 LG 50; cf. 2 *Macc* 12:45 Aquinas's stated reason that the souls are suffering doesn't make sense: people obviously can pray when suffering! The Catechism doesn't explain at all. What are the best reasons on either side?