I'm not sure that the lack of a seniority system is entirely linked to the poor t's and c's in the low cost sector. There are a number of other factors that come into it. At the moment in Ryanair as an FO your pay increases with your JAR25 hours and everyone is offered an interview for command upgrade at 2900 hrs.
Also in airlines like BA where you have a mixed fleet and an unstable roster (ie. not 5/4) there is more need for a system of seniority when you are bidding for days off, trips, fleet change etc. Almost none of this would be relevant in Ryanair. The only time some sort of seniority might be good would be when applying for annual leave and base transfers.