Every system is different and we can only relate to where we work. In the NYC Fire Dept. you have to be an EMT for at least a year in a 911 system before being considered for medic school. That's if the Fire Dept. pays for it and you get your medic through the academy. We are a two medic system so if your partner is the tech on that day he also does BLS and vise versa. This is where good BLS skills pay off and where I see all to often medics fail miserably. I know in Boston EMS even if you are a medic you have to work on a BLS unit first. We can go back and forth on this subject but it's all a matter of opinion and some will say go straight for the medic cert and others will say you have to pay your dues and work BLS first.
I understand what you're saying. I come from the same environment. For those that are unfamiliar with the NYC 911 system, the medics only get ALS job types. They're not getting the sick jobs, splinting injuries, boarding/collaring/KED on the BS Allstate-itis, to give a few examples. As such, the average medic doesn't see much BLS stuff. One medic I rode with back in the day was struggling with the board & collar. Others can't do a sling & swathe or a traction splint to save their life. In the NYC 911 system, if you didn't spend any time getting good at BLS, you'll hardly ever see that stuff as a medic. Chances are that if you do need to do some bleeding control or immobilization, you'll have BLS onscene doing it with you or for you. In this type of system, it's necessary to have some BLS experience going in to medic school. Remember, the ALS ambulance rotations are seeing the same ALS job types and the same lack of BLS opportunities. I did two years at my hospital as an EMT and then gained a third year of experience while doing the medic program.
Now, I've worked in two other systems down south. My last employer was a county run third service. The entire fleet was medic/EMT unless they were training medics, then it was double medic. The difference from the NYC system is that the buses down there get anything and everything. In NY the medcs can see several good jobs in an 8 hour shift. These southern systems run 24's. You might see a couple of "ALS" where they're okay, and you just do vitals, O2, monitor, IV, txp (V.O.M.I.T.). Your true ALS calls, the ones that NYC medics see on a regular basis, may only come once or twice a week, on average, if at all. They're also seeing all the BLS that the NYC medics would not. As such, an EMT in that type of system can and should go right to medic school. They'll see plenty of BLS on their ambulance rotations in medic school, and also see it afterward in the field.
At my current employer, we do a 16 week ALS internship. Our medic units also get everything, not just ALS. We also do EMD, so everything gets overtriaged, anyway. Medics here aren't thrown to the wolves, either. They can't ride lead on a medic unit until they have 18 months riding under a medic Lt. One could go BLS right to ALS here, too.
Really, in any system where every unit is medic/EMT, the EMT can go to medic school right away. The medic tells them what to do anyway, so they're really not learning anything. In NY the BLS often have to use critical thinking skills, any may have to handle CTD ALS pts on their own if ALS is extended.