You are right on that account.....the nelson plugs are a pretty good value
As I noted..... if a nelson type head is desired, one can be obtained with little more than an email or phone call.
Of note, the Jett SS40 pylon engine is sold in two variets.... the SEMPRA version with the std 1/4" glow plug, and the CAPS version which uses the nelson plug. From my experience with the two, I have seen no signficant rpm difference between the setups, plug wise. I usually run K&B 1-L or -HP plugs, or the Merlin 'hot' plugs jett currently ships with the engines. (Merlin plugs are outstanding glow plugs btw.... lots of differnt types and heat ranges) Side by side, the CAPS engine and SEMPRA engine run pretty much the same, just swapping the head button. The plug does not make a differnce, other than the Nelson plug will run for pretty much an entire season, where as I usually changed a K&B plug after a weekend of racing (usually nothing wrong with it...... the plugs go in my 'sport' plug collection).
(for those about to correct me, yes...the sempra and CAPS engines also have different size carb bore too, but I always run the smaller SEMPRA carb anyway)
Glow plugs actually control the ignition timing of the engine. In that respect, you can get a power band shift from changing from plug to plug...sometimes this helps, sometimes it will make the engine run like crud. Often you can achieve a higher top end, at the sacrifice of the ability to idle or transition well. Nelson plugs tend to fall into that catagory. The have a somewhat large diameter wire forming the coil, and they cool off quickly if you attempt to idle. Not very useful for a sport engine.
One of the other reasons the sport engines are generally not sold with Nelson plug provisions, is that the nelson plug installation does not use a gasket. If you overtighten a Nelson plug, you can destroy the mating seal, and the plug installation will leak. Most of us using them for racing, CL, and other uses know this, and understand the installation technique. The 'sub-average' sport pilot who tends not to read instructions would ruin the head button in no time. ... resulting in poor performance, and a returned engine and dissatisfied owner.... and the eventual bashing of a 'crappy engine' here on the message boards