I think people on here are getting confused with what "building a career" actually means. You say that moving to a jet is building a career, but how much of a career move is it? What is career building? Is it position within a company or financial reward, or both? You can build a career with Flybe just as easily as you can with any other jet operator. Assume you go to Easyjet, what career can you build with them? The exact same career you can with Flybe. With BA, you start your career on a jet, you finish your career on a jet. You can move around fleets, you can change seats, you can build financial reward, but you can do this with Flybe as well, but just not as varied and not for such a financial reward. If you joined Flybe and get a Dash rating then is it a massive "career move" to jump onto a jet with another company? It's a one step process isn't it, once you've done it, then what? Space Shuttle?
Flybe aren't in the process of getting rid of their 175s anytime soon so there's always a move to a jet within Flybe for now. If you join as an FO on the Dash you can build experience and seniority, your financial reward climbs every year and quite quickly, you can change seats to a command, you can then become a LTC, a TRI, a TRE, move to management. There's still a career to be had at Flybe, albeit not a massively varied one that you'd get with BA.
End of the day it's horses for courses, if you want a career where you're not bothered what you fly and you want to be home most evenings then join Flybe (but not at MAN or BHX), if you want a career that gives you an amazing array of aircraft, different sizes, long haul yadda yadda then join BA. If you want quality of life in the long run then go to BA (you won't get that in the short term though), if you want to live in the rejoins and not commute then join Flybe. End of the day the term "career" is very subjective and is an individual taste and not one person can dictate their circumstance onto someone else, but to say there isn't a career to be had at Flybe is plain and simply wrong.
Good luck with whatever you decide.
p.s. Time to command with Flybe for a fresh faced MPL will be circa 5+ years and don't expect a base in the South