Yay or nay?

Depends. If you look at ERA yes quite often. If you look at Whip OAV K/BB then no. 
 
Problem is they pitch so few innings 2-3 bad outings can inflate their ERA. I have had guys like that save around 30 games with a save % over 85% have an OAV around .220 a 6-1 K to Walk Ratio a WHIP around 1 and an ERA around 4.50

 They do take some extra managing, you can't leave them in very long and sometimes they miss a game where you needed them. Good control is very important for them

Yes, on a per-inning basis, due to in-game fatigue.  Including WHIP, imo.  Unless you shorten their pitch counts to 7-8 less than their stamina.