The "unsmart" feeling of busting is pretty boring, yes. However, team busting does suck. It takes every drawback of the medium and makes it far more noticeable.
But first, to counter your counter.
I don't know of many mmo's that arbitrarily increase a world mob's power just because you are in a group. Almost all of them have areas, usually instances, that are designed for small groups or raids. These are not the "open world" as the net would be synonymous to. The raids and instances of most mmo's would be more like the events that were once held around here, designed for larger groups of people to get together and work to coop an insurmountable force.
However, some Tower Defense games I have played do scale enemies based on amount of players, but I would consider this a far cry from a tower defense game.
1. Turn-turn around.
In a solo round you post, mod posts, you post again, etc. If you're lucky, you may get a couple posts a day. If you aren't, you may get a few a week due to busy/conflicting schedules.
In a team fight, you need to wait for two (or more) people to post before the mod goes. Sure, if memory serves there's still the 3 day rule, but by that time who cares? You might all have incredible timing and get a few posts a week, or you could all have horrible timing conflicts and get 1 or 2 a week. Even with the slight increase from throwing extra enemies at people that 2 posts a week (assuming the three day rule was called 2 times a week) still won't keep up with the several posts a week most solo busters enjoy because they have more to kill to start with.
2. Risk v Rewards
As loosely demonstrated above, solo busting is the "less risky" route to take for trying to get posts made. Which means you end up getting more faster.
Team busts take longer, having a higher risk of your battle stalling and taking forever, and you get a few fxp as compensation. Your battles are scaled "to difficulty" so that rewards can be kept the same as if you were solo busting.
I'd propose simply increasing the rewards you get for solo busting. Make rarer chips drop easier. Extra bugfrags if one of you has a pet. Maybe make harder enemies spawn in group fights with a chance to get their chip at a place unavailable to solo busters. Make team busting look as good as solo.
Crosses are nice and all, but they aren't something you can rely on all the time like 20 extra HP or one extra heal chip or enough room to slot that last piece of your ultimate defense program setup.
And lastly, the simple fact is this - if team busting was worthwhile compared to soloing, people would choose to do it. Since only 4? people choose to do it out of everyone here, I'm fairly sure the numbers would say that it's missing something.