I've motorpaced for years; since I've started using a PM, I have a much better sense of what it does, and what it doesn't, do. Looking at power profiles from motorpaced and race sessions, I don't see a whole lot of difference when talking about TTs. What motorpacing does do is simulate extended race efforts in a way that's highly motivating, albiet somewhat dangerous. Yes, I can set a target wattage on my PM, but I also know that ultimately I am the weak point in this system (I always can back off, if I'm not disciplined, etc.).
Motorpacing introduces an element of competition that makes me more motivated to ride at a higher level and for a longer period of time for more sessions during a training cycle without causing me to worry about burn-out. For me, it takes away some of the mental strain of training at very high intensities multiple times per week. Now that I pretty much only do time trials, I like having a carrot to chase over rolling terrain, which forces me to sometimes exceed my thresholds and requires me to consider how to recover. Solo competitive TTs are good, but when riding behind a motor, I'm always going to have someone a bit stronger than me to pace. Without the motor, I don't have anyone around that can ride hard enough, long enough for me to use as a pacer.
I regard motorpacing like any other training tool--I understand that for TTs, it primarily is a motivator that lessens the mental strain of intense training efforts on a repeated basis; I also know that my power profiles won't be much different than if I ride alone. But because the mental component is addressed, and because the motor can be unforgiving, I'll get a much better workout. For much the same reason, I'll use an ergo most of the time when riding indoors, as it limts one of the variables that form an effective session.
That said, before I got hurt, I was pretty much a pure sprinter; I'd motorpace as a way to practice my jump when going 40mph; this sort of training only can be done in a practical and controlled manner behind a motor. In this case, the motor simulated the conditions of an actual keirin or finishing sprint.
German Motorpacing on Velo (pre-UCI restrictions on wheels):
Typical Bicycle Designed for Motorpacing (note the small front wheel and reversed fork, which enables the bike to draft closer behind the motor):
Motorpacing Women's Keirin ((post-UCI restrictions on wheels):
In response to the question, "Should I motorpace?"
I used to do a good amount of motorpacing outside behind a small scooter. The thought at the time was that motorpacing would get the rider accustomed to riding at a race-pace intensity and speed. And it did work toward this end, but not necessarily for the reasons most people assumed.
One can get the same benefits from motorpacing by doing simple intervals on one's own, whether they are steady-state intervals or variable effort intervals (eg, sprinting around the pacer, and then falling back into the draft, etc.). In other words, there is nothing magic about sitting in the draft of a pacer, whether it is a scooter, derny, or a fast moving pack. That a pacer *does* do is provide a carrot, a form of visible motivation that might make it easier for some riders to ride at a higher level of intensity or for a longer period of time than they might otherwise do on their own--nothing more.
What I've been doing in recent years is to shift my motorpacing sessions indoors onto the Computrainer, where I can do it in a safe and controlled environment. I'll set a virtual pacer to perform at a high wattage level and then draft off of it, using the 7 to 21 foot draft window behind the pacer as my motivator to stay in contact. These days, there really isn't much of a need to pace behind a vehicle. We know a lot more about power and performance, so why engage in a risky--and in some locals illegal--activity when there are other, more effective options available.
Hope that this helps.