Most of my shoes have both systems (Lake). I see no drawback to having both. As long as the shoes have the system I need, they are good in my book. I do use both systems. 4 of my bikes are either LOOK Delta or Exustar track slotted (traditional) cleats; both using 3-bolt pattern shoes and one bike has SPD. Having shoes that do both is only a plus.

I don't see the ability to change cleat types as a big advantage. For my knees, cleat position is so critical that I will not cavalierly change cleat types. Two different systems call for two different shoes with the appropriate cleats permanently mounted. (When the cleats wear out, I tape the circumference so I can get the replacements in exactly the same place and rotation.)

Ben