Most people don't have the cash in hand to pay the comission since we live in a "how much can I afford" society. If you bury it in the house price you can finance it. It also allows the whole line of how a buyer gets their agent "free."
Math isn't as important as good sales pitches to most people.

Lynne, That is an excellent question. I believe the best answer would be for you to consult with your accountant first and foremost. Every person's individual situation is different depending on their own scenario, but you may be surprised (plesantly even) once you speak to a qualified professional about how that scenario might impact you.

My question to you is why would you, as a buyer, pay any real estate commissions anyway, when usually they are paid by the seller? If you are concerned about paying the taxes over a 30 year period, maybe you could have your real estate agent negotiate the offer on your behalf with that scenario in mind, which could put you in a better position when you do your taxes by incurring some real estate expenses at purchase. Of course speak to a qualified accountant first - not all accountants are created equally either. Make sure you speak to one that specializes in real estate and/or at least owns investment property themselves.

Good luck to you!