I'm a jian practitioner and I consider myself to know a real jian and jian techniques. However, it is hard for me to compare jian and rapier since I'm not a rapier practitioner. Therefore, I will not take an opinion on a practitioner of Renaissance fencing as a fact/truth. Since they might not know anything about jian, in fact I think only a very handful people in this world know a real jian technique.
About your question, where's the shield? My answer is "
jian has no shield". Jian is a weapon that does not rely on a shield at all. The jian guard (or jian shield) is not used as a guard or shield. In a real jian technique that I learn, it is used for jian manipulation which make jian very flexible in manipulation & creating a unique jian technique/strategy (if you put the rapier shield on jian, this technique that used for this new jian has to change too which might not be good as the jian without the shield). In the real jian that I know, the edge of the guard should not point to the tip of the jian for a practical reason of the jian usage. If you go in most martial art store, most (if not all) of jian guards, you will see, are pointed to the tip of jian which is wrong. My master told me that, in the old book about jian, it was said that jian has three tips point to the direction of jian handle (i.e., 2 tips on jian guard and the end of jian handle point in opposite direction of the tip of jian) - in other words, this means the guard should be widest near the hand and slant inwards towards the blade. However, the jian that used in ceremony has jian guard that point to the tip of jian because it will create 3 tips point to the sky (or heaven) and it may look somewhat prettier - but will create a flaw when this jian is used. If you are interested to learn more about jian structure, I refer you to an article by Adam Hsu, "A Straight Talk about Straight Sword" in Kung Fu TAI CHI magazine issued in Febuary 2005.
So, why jian has no shield? This is because jian will not be used to directly block the opponent weapon and it is a weapon with refined skill of its user. In fact, you may consider that "
jian blade is the real jian shield" (and different part of jian blade have different type of usage - don't treat the whole blade is the same). Jian "block" (when I say "block", it doesn't mean block but some technique that used to prevent opponent weapon reach jian user) with the blade but it is not a direct block - this "block" is created through a circular movement (which may not the same speed - just forget anything that you learn from Wuxia novel & TV/movies). This circular movement is one of the key points of the jian. It could be a jian shield. It can be used for entangling the opponent's weapon. It can be used for both offense and defense. Also, a high level jian user could use jian with almost no contact with his/her opponent weapon at all through his/her footwork & the way he/she manipulate weapons. So, if there is no contact or no block, why we need shield? Right? (Note: sabre's shield is a necessary & important part since block is a major technique of sabre)