I play in a 16 team 6x6 using holds and holders generally go several rounds later than closers. This happens for several reasons, including the fact that holders are not as stable as closers, holders are more numerous than closers, and holds are not nearly as concentrated at the top end as saves. For example, there were about 25 guys that had 20+ holds and a similar number that has 20+ saves. But the leading saves guy had more than 50 saves, while the leading holder had just 36 holds.
So, while holds are just as important as saves, holders are not as valuable as closers, because holders are less predictable and holds are less concentrated in a small number of holders.
Holders generally start to go at about round 13-15 in my league.