If there is any time where only the medication is running, then at that point there is only medication in the line (say, going at, for sake of illustration, 4cc/hr, and the tubing holds about 8cc so the tubing is 100% medication and 0% bolus fluid). Let's pretend it's blue, so you can see it well. It's blue all the way down to the vein.
Then, upstream, the bolus pump kicks in at 150cc/hr, and in an instant two hour's worth of medication essentially gets pushed (or more, depending on how much of that tube was full of blue before). This could be an exceptionally bad idea for many meds given as maintenance drips, like hemodynamic meds, sedation, or electrolyte replacement.
After that push, of course the medication is being flushed in at its set rate along with the bolus. Since it's along for the ride with the bolus fluid, the concentration the medication is quite low, although the absolute amount of it going into the patient is accurate, 4cc/hr. Your tubing is now very pale blue, right? That 4cc little bit of blue diluted by the bigger amount hourly of bolus, but at least it is received at a constant rate.
Now, however, the bolus is shut off. The tubing is still mostly bolus fluid, with a little bit of medication in it, the palest of blues. However, now it's only running at 4cc/hr, the rate of the medication drip up above, and there isn't much medication in those initial cc of fluid in the tube-- it's mostly bolus fluid, so for some longish period of time the patient is not getting his maintenance dose of medication, in this example, about two hours. Also not a good idea for any med meant to be given at a constant rate.
Not until the blue has had a chance to come down and fill the tube all the way to the vein is the patient getting his regularly-scheduled dose of blue stuff.
This is why you need a double-lumen line. The blue stuff is in one and is never pushed, flushed, or drawn from, to maintain a steady administration rate. The boluses run through the other lumen (or another line entirely), being locked or TKO in between boluses.
Hope that helps you visualize why you never run boluses on top of med drips.