I hear a lot of people are starting to get fed up with this show, but I think it is getting better and better. If you check out page one of this thread, originally I was pretty harsh on the show, and the sheer amount of time travelers was one of the reasons. In the films each side only sends one soldier back, it seems like if it were an easy process, they'd just send a whole army back to whipe out John and ensure Skynets safety.
Now that we are getting into all the alternative timeline stuff, I think it is getting pretty fun. T1 and T2 give us the theme, there is no fate but the one we make. T3 (which I hate with a passion) screws this up by ultimately saying the outcome of the future is already set, you can dink around with it and you might change the players involved and the method, the date and time, etc., but what is suppose to happen is going to happen no matter what you do (similar to Lost's paradox free philosophy on time travel, "the timeline has a way of course correcting itself").
While I like Lost's take on time travel, you can alter the way things happen, you can postpone them, but you can't prevent it from happening. I like TSCC's branching realities approach even more, and I think this fits much better with the first two films. In TSCC it seems by going back in time you can alter any number of things and make them happen differently. We have Jesse remembering things that happened to Derek in the future, which Derek has no recollection of because they are from two completely different time lines. Derek is from the future following the films. Three major things happen to severly alter Jesse and Riley's future from that of Derek's. First, John sends Cameron back in time to protect himself, she kills the evil terminator, and sends them into the safety of the future (obviously for nothing more than for ease of story telling). Second, Derek believes if he and his men go back in time with a list of the names of men responsible for Skynet, then they can merely assassinate them one by one, preventing Skynet from becoming a reality. Third, Skynet is also doing its fair share of meddling with the time line, but we don't have much detail on what exactly they are up to yet.
In Derek's future, Cameron was a captured and reprogrammed terminator, something we learn John was getting into in T2 when he trusted Arnie enough to send him back to protect himself when he was a kid. In the resulting time line, John probably had much more sympathetic feelings for Terminators due to the strong bonding that went on between him and the T-800 that he sent to protect himself. As we see from Derek's reaction to all the terminators running around the base, many of the resistance fighters are uneasy with John's trust in the reprogrammed enemy soldiers, and it was already a misguided trust that cost some of the resistance fighters their lives ("sometimes they go bad"). In Jesse and Riley's time line, Cameron stays with John and influences him from the pilot of the TV series and on into the war against the machines, very likely resulting in a John who is even more sympathetic toward the Terminators than T2 John was. As in Derek's time line, this is a source of discomfort for the resistance fighters, and as mentioned by Jesse, something that is costing many resistance fighters their lives, or at least she sees it that way. I am very interested to see how they continue to make this play out, sure it could be better done than the writers are managing to do it (I think this is inherit with short deadlines, numerous writers, and overall story arcs with poorly planned details, as can be seen in other shows like Heroes and Lost), but in this day and age of crappy TV shows and movies, this one is at least very entertaining and takes place in a universe of a franchise I have been a fan of for a long time.
Throughout all the time travel going on, it is very possible that Skynet was able to alter the time line by carrying information back in time to allow them to better develop means of time travel, and as a result are able to send more soldiers back with greater ease than back in the days of T1.
On a final note, many people often point out how ridiculous T1 is, since the whole plot is a paradox. If Kyle was not sent back in time, then John would not be born, and if John was never born, he couldn't send Kyle back in time to impregnate his mom. I have always realized this was true, but it never really bothered me too much. However, while a linear time line makes this idea very silly, the idea of alternative futures and branching time lines actually allows for this idea to work perfectly. Imagine if in a much earlier time line, the resistance fighter Kyle is sent back for some other reason, perhaps to stop the war all together, during his mission he meets Sarah, and the two fall in love and hook up, during his time with her he tells all about the Skynet and the future war, then somewhere along the line he is killed. Sarah raises John, telling him stories about the resistance and the war with Skynet. John grows up with the knowledge this is going to happen, and when it does he is ready to rise to the occasion and ends up becoming a great leader in the resistance. When we eventually get to the branch in this time line where Skynet sends a T-800 back in time to kill his mom, this John realizes that it has to be Kyle who goes. Then Kyle tells Sarah his son becomes their great leader, she goes through the trauma of nearly being murdered by a terminator, and sees the man she fell in love with get murdered, she becomes the militaristic mother we know from T2, who feels she must ensure her son becomes this great leader, which as a result, as seen in TSCC, we have John doubting himself and feeling a lot of pressure of becoming something he doesn't feel he can become. If we don't assume T1 is the original unaltered time line, then everything fits together rather nicely.