Vacancy Rate is in the 12-14% rage for the CBD. That's a relatively healthy rate and has very slowly declined from a high of about 20% in 1995 (while overall Class A space has increased by roughly 40%).
I believe the concern is that most of the absorption of new office space has been tenants moving from Class B up to Class A space, or Class A to less expensive Class A space. A lot of shuffling around and very little "new" office tenants moving to downtown or exisitng companies growing into new space downtown.
In addition, office rental rates downtown have historically been and remain to be inexpensive ($18 sf, the same as they were in 1999). This is good for the tenants, but not the developers. Greenville developers have always built downtown for margins that most developers in other cities would not be interested in.
So, if we have multiple 14-20 story buildings being constructed, this sends a very inaccurate message to developers that our downtown office market is booming when it is actually quite healthy, but not fully robust.
And I'll just throw out there that I personally believe that a 14 or 20 story tower downtown would not really fit the wonderful scale of downtown. I believe one of the greatest attributes of downtown is the 'feel' of livability. In this I mean, Main Street (and the side streets) are outdoor rooms with the facades of buildings as their walls and there are no towers that interrupt that scale. The Daniel building is the tallest building downtown (right?) and it pretty much demarcates the end of the 'walkable/vibrant' area of Main Street. When Mr. Daniel built that building it was an incredible gift to Greenville when very little was going on downtown, but now that it is the showpiece of a metro of 1,000,000, the city and developers should consider the impact of towers on downtown and not just build up to 20 stories tall b/c they can.
(Sorry, will hop off my tee-tiny soapbox, really enjoy this discussion board)