Pros:

• A travel professional should be familiar with all the restrictions, change fees, cancelation penalties, etc. associated with airline tickets and hotel bookings. These can be overwhelming, to say the least, for a novice traveler, and even an experienced traveler might not want to take the time to stay up to speed on all the terms and conditions.
• A travel professional will have a broader experience base when coming up with flight alternatives, suggesting hotels, etc. Even for areas they're not a familiar with, they should be able to find out the basics so that your employees don't get stuck with less-than-suitable travel arrangements.
• A travel professional should know how to find find better rates for reasonable options than an inexperienced traveler would be likely to find.
• Having a good travel professional can make it much easier on your consultants when they need to make changes "on the fly", at the last minute, and don't have the time to worry about these things themselves.
• Booking travel through a central person or agency can make it easier to enforce travel policies, rather than finding out about problems when employees submit their expenses after the fact.
• It may make sense to have someone with travel planning expertise spend the time necessary to make travel arrangements than to have employees (whose specialty presumably isn't travel) taking time out of their schedules to do so.

Cons:

• A travel professional won't be able to tailor travel plans as well as an individual can for themselves. For some travelers, this is fine and they're happier being less hands-on with their travel arrangements. But for others (like me), this can be a real pain. Personally, I'd rather see all the options and weigh them myself, instead of doing a lot of back-and-forth with a travel person to work out all the possibilities.
• For people like me, it's not a lot of fun to have to go through an extra layer of overhead to arrange something you could do yourself.
• Depending on your employees travel patterns, a travel professional may be an unnecessary expense. If a lot of your travel is to the same locations, you can probably figure out amongst yourselves which hotels make sense (i.e. word of mouth), and it's generally not too hard to get a pretty good airfare deal on Expedia, Orbitz, Travelocity, etc. these days.
• Similarly, if you're willing to (or need to) buy full-fare tickets as a rule for maximum flexibility, you may not need much help from a travel professional.

I'm sure there are plenty of other considerations, but those are the ones that jump to mind from my experience.