Depends on the AIO.
A quality AIO that's 240mm or bigger would keep your temps in check, and allow the 2600 to boost to it's max potential more often.
All depends what you're comparing though. A smaller 120mm AIO would not be worthwhile IMO, and, equally, an AIO is not the be all end all. A quality air cooler (Noctua NH-D14, 15, or S variants, larger be Quiet! units etc) will rival a 240mm AIO anyway - although the AIO is not as bulky.
Does the 2600 "need" it though? No. The stock cooler is sufficient for stock operations, just may not boost as high.