A professional should NOT skimp on good equipment. They make money with their stuff, and like in most all businesses, their equipment is merely a tool... a tool that gets upgraded regularly.
I would say that if you are pro, get the best lens out there for your business, you will see the difference in your pics and in your client's level of satisfaction.
HOWEVER, the OEM manufacturers are not *always* the best. I am not a Canon guy, but I can give you a perfect example from the Nikon world...
In the 17-55 range, we have 3 main competitors, The Nikkor 17-55, the Tokina 17-50 and the Sigma 18-50 (there are 3 models of this lens out, and on that basis for this conversation, I am referring to the Sigma 18-50 EX DC HSM Macro F/2.8).
Several leading photography magazines tested these lenses in a shootout and in ALL CASES, the Sigma was sharper, had less flare, lower CA than any of the others. It was also 1/3rd the price of the Nikkor, and offered a 3:1 macro... something none of the competition offered. In this case the "best" lens was not a Nikkor, nor was it the most expensive one either.
To globally say that OEM lenses are aways better than aftermarket ones means that you are walking around with horse blinders onyour face and possibly missing out on some fantastic glass at affordable prices.
I am not saying that this is always the case, but YOU, as a professional, need to do this very homework and find out what is the best for your needs and the needs of your clients.
You can play safe and always go for Canon "L" glass, it is known good... and as a pro, I think it should be your FIRST consideration, but certainly NOT your only consideration.
If you cannot afford the best lens now... rent it, and purchase it when you can, or take out a business loan if possible.
I do not think you will find anyone argue the fact that using the best lens possible is always preferable to using the #2 option. Your pride and desire to be the best you can be as a professional should demand no less.
In the end, you will decide how serious you are and how far you want to take your photography, and your choice of equipmentwill reflect this.
Good luck in your choice.
