I have a few thoughts regarding mines. First, I agree that mines as they are in this game just don't work as they are and need major reconsideration. I have thought long and hard about this and this is what I came up with.
1. Mines in space should not be used like today's naval minefields. It works fine for mining narrow straits and harbors in waterways on earth, but in a 3D space (even though Sins is not 3D) "traditional" mines just wouldn't work. I think that the way mines work in "Banner of the Stars" (relatively recent anime sci-fi series) are the perfect way to go. These mines are basically warheads with short range rocket motors. They drift in space until a ship comes along and activates them. Once activated, the rockets fire, the mine becomes basically a guided missile, and the mine rams the target and detonates.
2. To prevent spamming, I think that there should be a new defense structure called the "Mine Control Center" or "Planetary Defense HQ". This structure would basically be a small outpost that players would build to establish, control, and maintain the minefield. Each structure could use a fair amount of tactical points and only support about 20 or so mines (to prevent spamming and lag by having too many onscreen). Before I hear anyone complain, let me remind you that if mines are done like the above suggestion, you could get more bang for you buck (especially if they were made slightly more powerful).
3. To counter the fact that you have a relatively small amount of mines, you could make it an option to set an engagement zone for this structure like you do with fleets. This adds a new tactic to the game. Do you have a small, compact minefield thats more devastating to a given area, or do you have a larger, spread out minefield that covers more area but may not cause as much damage in a given area?
4. This type of defence platform could also have a reason behind it. A control center could, for game fiction purposes, deactivate the mines when no enemies are present preventing "friendly fire" from acidental collisions (using IFF codes and such could work, but what if someone's frieghter is out of date or something and has the wrong code). When enemy ships are inbound, the "Mine Control Center" or "Defence HQ" transmits activation codes to the field and the mines become active, with the IFF codes helping to further prevent "friendly fire".
5. These structures would automatically replace any used mines, eliminating micro-management. To make it even more valuable, this structure could also be upgraded with a missile pack or flack cannon giving it the ability to defend itself against minor threats if the mines were not enough. It could also solve another problem: having your newly conquered planet surrounded by hundreds of mines with no (currently) effective way to get rid of them. Destroying the structure means that no one maintains the field and it becomes inert/self-destructs.
The current problems with mines worry me because the possible spamming and lag by having to many onscreen could give me my only forseeable reason to not enjoy this game as much as I do. I really like this game, and I want to help fix this problem. I look foward to hearing any comments or suggestions to my idea.