I can't speak to Delta, but today DevGru (ST6) operators have rotated in and out of other SEAL Teams many times. Most SEAL platoons could be called upon to do most ST6 missions if time was critical. The guys at ST6 would probably argue otherwise. But it gets back to the old military maxim about a 90% plan done now always being better than a 100% plan being done too late. And now we're in a war, so casualties are acceptable. That means that if a ST platoon takes some casualties that a ST6 or Delta crew might not have taken, so be it. There's a war on. You can't always wait for the Jedi Knights to arrive, because the target will be gone.
The Army actually has policy in place where a subset of SF units is trained in certain tasks that are normally the province of other units. The point of this is in war or peace to give the combatant commander a handy tool that's right there in his toolbox.
The guys doing specialised direct action as the main thing on their dance card are way better at it than the guys doing UW, but the UW guys can put on quite a firepower demonstration when required, and everybody comes to each other's help when the fit hits the shan.
Incidentally, a lot of the SEALs are working in very small teams, sometimes with a NSW EOD guy, and they tag on an op when it looks like fun, and go somewhere else if that's where the action's gonna be. They are having a freelance war! And everybody is glad to get them, and sorry to see them go (if only because you're down by three long guns and it probably means the hunting is better somewhere else).
d.o.l.
Criminal Number 18F