Perhaps he is worried that you are going to spend a LOT of money on equipment that you just don't know well enough. Every experienced photographer knows that a TC on a wide angle = no go. And well, not to be impolite either, but it seems that you really don't know that and thus you are not very experienced. And if you're not experienced, why are you then buying all this expensive equipment if cheaper equipment would suit you just as well?
We're worried that you might be wasting your money on equipment that you won't use to its full potential.
It's not that we judge how and on what you should spend your money, but we are hoping that you spend your money wisely. We've seen just too many people here buying a 5D with a 24-70/2.8, thinking that every picture will be beautiful now, and then they come back and complain that their cheap Ixus takes better macro shots than their $ 3000+ DSLR set.
If you are willing to be content with just 'ok' pictures, (eg. by using a TC on a 24-70L ), then why not get a good 'ok' lens (like the 28-135) and spend the difference in money on other lenses (eg. fast primes or a nice macro) or more accessories (a good flash).