It made sense at the time because Labonte's sponsor, General Mills, signed on at RCR to sponsor a 4th team at the operation and Labonte had a contract to drive 15 races for RCR in the Nationwide Series. Eventually, that rumor fell apart and Labonte re-signed with Petty for four more years.
Thursday, however, those plans completely changed again when Labonte and Petty agreed to amicably part ways immediately -- leaving the 2000 Sprint Cup Series champion without a ride for the moment in 2009 and continuing a nasty downward spiral of Petty Enterprises.
Petty plans to still run, at a minimum, a single-car team in 2009 but doesn't yet have a driver lined up.
Looking back, I can imagine that if the opportunity was available for Labonte to move to RCR like the rumor said, he's probably kicking himself slightly right now. However, we don't know for sure if such an offer was on the table and you surely can't fault Labonte for his loyalty.