It would benefit the Cardiff region if more jobs were accessable to people in merthyr and aberdare (assuming there were any jobs in Swansea).
Things like that help develop the local economy. The transport infrastructure we have in south Wales has basically grown out of a system that was designed to extract (mineral) wealth and get it out of the country as quickly as possible. Other parts of the world with a similar transport infrastructure (several examples in south America I think) are often economically worse off than their neighbours. Anything that makes our transport infrastructure not look like that would probably help.
It is similar in North Wales too.