Even if there was relegation in Pro 12, if you look at the record since 2005 when it went to its current format as regards the Welsh regions, Connacht is the only Irish team that would have been relegatyed (2008, 2009 & 2010). Since 2011 it has always been Italian teams on the bottom - Aironi (2011 & 2012) and Zebre, who are just Aironi in another form, in 2013.
But the point here is that there ARE only four Irish Teams, because there are only four Irish provinces, there ARE only four Welsh teams because there are four Welsh regions. By default, they are the best teams in those countries. If an Irish or Welsh team finished bottom of the Pro 12, and was relegated, who would be the team to replace them when there isn't one. There is no tier below the Pro 12 from which to promote a team. In fact it is meritorious indeed that teams like Leinster and Munster, who have performed well and won the HC on several occasions, have never been near the bottom despite that fact that they supposedly rest players because there is not relegation.
The "fight for relegation story" being put up by the PRL is a crock of shiite. I would argue that if the Pro 12 teams didn't rest players regularly, then tables will still end up looking much the same. In the last eight years, Leinster have finished 3rd, 3rd, 3rd, 1st, 3rd, 1st, 1st & 2nd; while Munster have finished 4th, 4th, 6th, 3rd, 1st, 4th, 3rd, & 6th. These are all qualifying position despite supposedly playing weakened teams for some matches.