I hear this a lot, and frankly is bad advice. First, having experience in business jets gives you something to fall back on if you get furloughed or laid off, which if you look back at the past 20 years is almost a certainty.
Second, while 121 time is relevant to employment at other 121 carriers (and some may even find it preferable), I am not aware of any airline that requires 121 time. Jumpseat on any airline and you will find pilots of all backgrounds. In fact, many airlines prefer a diverse pilot corps. My employer loses almost all of its pilots to the airlines, including Southwest, Spirit, Atlas, etc. Few have any 121 time.
Third, having part 135 or business jet experience will not bar you from employment at a regional if you decide 135 is not for you. To the contrary, it will put you far ahead of the CFIs with 50 hrs of multi time. However, many part 135 carriers are hesitant to hire 121 guys out of fear that they will jump ship they get called by a major. The exception is if they have prior 135 or type specific experience, particularly as PIC (it's always hard to find experienced captains).
Fourth, part 135 and biz jet flying is invaluable experience that you will not get at an airline. Equipment is older, you don't have someone planning your flights, and the flying is more challenging and demanding as a result of varied destinations, difficult airports and terrain, etc.