I would say yes: it's worth building additional retirement savings on top of a defined benefit pension plan (plans that pay set annual income). Here are a couple of reasons:
Don't put all your eggs in one basket. While OTPP is probably in good shape, things can and do happen to pension plans. While there is a provincial system in place to guarantee some of your pension income ($1000/mo) if your plan goes bust, your benefits are not 100% guaranteed.
Defined benefit pension plans are designed to provide recurring annual income, like your paycheck when you are employed. You can't "take more out" from your defined benefit pension plan when an emergency comes up. Whereas, your RRSP (and eventually RRIF, in retirement) are accounts from which you can take out extra in any given year, if necessary.
That being said, Canada Revenue Agency (CRA) won't let you save as much in your RRSP as other people who don't have pension plans:
Normally, individuals in Canada are entitled to save 18% of their earned income, up to a limit ($21000 in 2009) in an RRSP each year. However, to level the playing field, individuals who are in a pension plan get a "Pension Adjustment" (PA) number on their T4 which reduces their available RRSP contribution room. Otherwise, they'd be able to tax-shelter more income for retirement than others.
So, I would suggest if you have the RRSP room, consider using it. I'd also suggest you look at a Tax Free Savings Account (TFSA), especially if you don't have much RRSP room due to the pension adjustment. If you're not sure whether to use an RRSP or a TFSA, consider both.