Must be a very good starting rotation given they are in the same division as the Giants, Dodgers, Rockies, and Diamondbacks. Typically, when a team says it would rather a guy be a reliever than a starter, it's for one of two reason: The kid does not have the tools to be a starter or they are one or two relievers away from being a contending team and need the help from a strong arm. Not sure which is at play here.
Or three, they just don't get it - you NEVER have enough starting pitching. With the corollary, you never waste potential starting pitching on a one-inning bullpen "bridge" role. Whatever the attributes required of an "8th inning man", I am guessing the Padres have a lot more options to fill that spot from in their system than they do (possible) MLB rotation starters.
In most cases, it is your reason #1 at play, or should be. In the Padres case, IMO #2 is
seriously deluded wishful thinking at best.
All this with the understanding I don't know Cashman specifically, just what I've read/heard.
An interesting question occurred to me, though. The Padres play in a park that - relatively speaking in the context of today's game - severely represses offensive stats. For that reason, do they take their pitching a little bit for granted? I don't know. But it reminds me of following the Astros back in the Dome years, especially when the Dome was notoriously detrimental to offense. I don't know about the club, but as a fan, I certainly took the pitching for granted ... or at least took it for granted the Dome would dramatically help the pitching.
The Astros could bring in a guy who might have been so-so elsewhere, and he'd pitch better, sometimes dramatically better, for the 'Stros. In some cases, the Dome/road splits wouldn't be that different, but the pitcher would have much improved results overall after putting on the orange and blue. My uneducated guess was that pitching in the Dome built up a guy's confidence, which boosted his confidence everywhere he pitched.
I was going to use Mike Scott as an example, except in his case I believe it was mastering a killer pitch after he got to Houston that played the biggest role. One guy I remember specifically is the guy who replaced Scott in the rotation when Scotty had to miss a couple of starts, in '89 I think. A humpty who'd been getting his ass handed to him in the AL; the Astros picked him up in the offseason from the Twins for a minor leaguer. Ladies and gentlemen (and the guys who post here), meet Mark Portugal. I specifically remember thinking at the time that Portugal would be a better pitcher with the benefit of the Dome; although I had no idea how much better. Basically, he saved his career in Houston.
Howe plugged him into the rotation when Scott was hurt that year, and then left him in after the team released an aging and ineffective Bob Knepper; and Portugal went 7-1, and then went on to be a reliable starter in Houston for several years after that. And a Giant killer, as I recall, back when the Astros were usually chasing SF in the old NL West.