Soul
Alright, so let's break this down.
Contrary to common understand, which player has guild lead does not affect the total score of the guild a lot. It does affect the invividual score significantly, but the added up guild score does not get as affected.
Let's say you have a bad run and you score 5 kills, and your guildmate scores 15 kills. In this situation, you would commonly be getting 10 points, and your guildmate would get 30 points. If you had guildlead, you would have 15 points in this situation, and the other player would have 45 ponts. Individually, improving your score from 30 to 45 is quite a boost, whereas the total guild score is not affected as much, since you have to subtract the number of points the guild leader got. So, in this case, the difference would not be higher than around 10 points for the overall guild, even though it would affect the individual player with lead quite a lot. This might be decisive for the MvP, but it will not change which guild crowns most of the time.
Obviously, 5 kills versus 15 kills is a very extreme situation, and the difference isn't very big even then. I would say the difference between a player having a good and a bad run is usually around 5 kills, in which case you would only be able to get another 5 points out of having guild lead on the right person.
That said, something we have to take into consideration is the guild-lead swapping for extra points. As most players who siege regularly might realise, it is close to unrealistic to swap guild lead to somebody just as he's about to get a kill. Even if you manage to do it, you'll only be rewarded by one point. Given my previous argument, the overall impact wouldn't be as big as it might seem. I would say you deserve those extra 5-10 points if you manage to synergize the passing that well. It is not realistic to assume this will result in having 2 players have lead every siege.
This all being said, I would definitely say I support this suggestion. It would add an extra touch of team synergy to the siege, and the overall difference between a guild swapping lead efficiently and a guild not swapping lead at all wouldn't be close to as big as people might think.