If you're talking purely in terms of dollars and cents, then it USUALLY isn't a good idea. However, there are people who do come out way ahead in the game...for instance, those who paid a few hundred bucks for a resale SDO 2 bdrm, deposited it into II, and then exchanged each side for a 2 bdrm in Hawaii or HRA. There's no question that these people are coming out ahead.
Generally speaking, if you want to try and play that game, you have to be flexible with your travel dates, able to travel during shoulder and low season, and willing to spend a lot of time searching II inventory for the best deal.
If that isn't you, then your gut is correct. It probably makes better fiscal sense to simply rent, especially since Starwood MFs are some of the highest in the industry, and the economy is still sluggish, which means many owners are content to try and rent out their units for the cost of MFs. There's very little incentive to buy now when you can rent the following*:
Marriott Summit Watch (Park City) 2/2 ski - $3000
Marriott Grande Ocean (Hilton Head) 2/2 OF summer - $1800
WKORV 2/2 OF June- $2900
Marriott Maui Ocean Club - Napili 2/2 OF summer - $3000
HGVC at Hilton Hawaiian Village (Oahu) 2/2 OF - $2800
Royal Cancun Club 2/2 OF many weeks - $1000
Marriott's Grande Vista (Orlando) 3/3 - $1850
Sheraton Vistana Villages (Orlando) 2/2 - $1000
Marriott OceanWatch (Myrtle Beach) 2/2 summer OV - $1800/OF $2500
Westin St. John Bay Vista summer 3/3 - $3500
Marriott Aruba Surf Club winter week 2/2 OV - $2600
Hyatt Pinon Point (Sedona) 2/2 May - $2200
HRA summer 2/2 fall $1600 summer $3500
*all current or marked as rented on Redweek