I've done so many times, but they break out to only three circumstances (and one situation sorta beyond my control):

1. I *love* Starwood (SPG) points! They are truly the "stem cells" of the points and miles world, since you can convert them into so many programs. I have used Starpoints on a couple of occasions to "top off" an account in another program, in order to get an award. Indeed, by doing a triangular trade (long ago), I managed to turn SPG points into HHonors points by converting them to Delta SkyMiles in order to have enough SkyMiles to do a special redemption to HHonors points (but that's not an efficient use of Starpoints if done as a regular thing).

2. I have often redeemed Marriott Rewards points for travel packages that include 7 nights plus between 50K and 120K miles in a frequent flyer program (it was AA, which no longer participates, drat!). You paid a rate of one Marriott Rewards point per airline mile, and since if you worked it right you could get 20 Marriott Rewards points per dollar spent with Marriott, that meant 20 airline miles per dollar spent -- for those miles, at least.

3. Starwood has a similar "nights + flights" redemption program, where you can get a 5-night stay at certain classes of hotels plus 50K miles for much less than the total for an award of 50K miles and one for 5 nights. Again, in connection with an award stay I was planning anyway, I'd frequently take the special award to get the 50K miles at a reduced rate.

4. The "sorta beyond my control" transfer came when TWA was absorbed by American, and I had the option of having my hundreds of thousands of TWA Aviators miles converted to AA miles, or letting them evaporate. I chose AA, and those program miles (including miles already redeemed in the Aviators program) counted toward my Million Miler status with AA.