Combo edges are ok depending on your uses and how long the blade is. I agree with JD that they are pretty useless on a short blade. If you have a 3" combo then you can still do plenty with the 1.5" pe, but 1.5" of se just isn't much to work with. Could be better than nothing though depending on the scenario. I also believe that a lot of folks don't like the way they look so never even consider them based just on that.
Bottom line is that combos exist for a reason and though many (including myself) aren't huge fans, they have their uses. I could definitely see a maintenance/handy man who only wanted to carry one blade using a combo. Pipe, fabric, heavy tape, hose, drywall...there are a lot of things that a pe knife doesn't cut that well. Not everyone wants to go fully serrated, and not everyone wants to carry two knives (like I do). I'd say if you like it and see yourself using it, go for it.
Edit: this post got me thinking. I would like to see a combo edge with like 2/3 serrated and 1/3 pe. Call it a combo with a little extra se, or call it full serrated with a little extra pe...Whatever. i think it would be more functional than a 50/50 combo edge.