Good question Kevin.
The cheapest Open Variable on the market today is priced around Prime + .70-.80%. This arguement applies to anyone considering an Open mortgage (be it fixed or variable). Simply put, if you are not planning to or do not have the foreseeable capacity to repay the mortgage in full, within the term, then you are better off taking a Closed/Convertible term with some prepayment priviledges, as the cost of borrowing will be substantially reduced.
Example. TDCT is currently offering a 1 year Open mortgage at a rate of 6.55%, but their advertised "best rate" 1 year Convertible is at 3.75% with the actual street rate being substantially lower than that (2.66%). Their Open VIRM is at P+.80% whereas their Closed VIRM is advertised at Prime, but available at P-.10%.
This interest rate premium is too big a price to pay, even in the short term.
For those who are comfortable with the variable rate ride, why is now not the time to jump into a Closed/convertible Variable? Because the banks will charge you an interest rate penalty (3 months interest or IRD), if and when you decide to move out of your Prime - .10% and opt into a VIRM at Prime - .50% (for example). Why would anyone pay that penalty as you would effectively negate any advantages of the lower interest rate?