1) Cash payments are not equal over the life of the trust. Your scenario would result in substantial front end loading. Best case, were you to get $13.50 back the first year and $3.00 over the remaing life, the return would be a fantastic.
2) BPT units actually have two elements of value, future cash payments, as discussed; and also an option on the price of WTI based on the possibility of extraordinary price appreciation. The latter is virtually irrelevant today in my opinion, but was the sole value of Trust units recently when WTI was hovering at $10, on its way to $5 in the minds of many experts. BPT was then selling below $5 with no cash payments.
To understand option pricing, we could go off into a discussion of Black-Scholes, suffice it to say these gentlemen won a Nobel Prize for their contribution to modern finance.
In all seriousness, if anyone finds this puzzling they might give serious thought to Exxon or the like. You'll still be subject to the vagaries of future oil prices, but in a vehicle managed by humans who can react to future change which is not the case with BPT.