I agree. A trapper is more essential than a bowa and windy at least.
It depends on what your team is lacking. A team that consists of a BvC, a Bonemancer and a Hammerdin has very high defense but lacks a bit in the offense. However, the best result is always the one where all team members can stand alone versus most opponents. A Trapper is pretty much toast if facing anything with 85%-95% Lightning Resistances with a ranged attack or massive life.
Not to mention that a trapper has relatively similar matchups against the classes that a windy is normally used against, being paladins, certain sorc builds, and other sins. Being able to mb and double team for a quick kill is too good an option to pass up.
Double-teaming is all fine and dandy, but if you think about it, the average damage of a trapper is quite low. Yes, they do have 10 blasts and 5 traps, but the average damage of a single trap, even at 14 000 damage is a mere 7000, meaning 8925 for the whole load at 85% LR. That's the problem: the reductionable damage and the slow applying of higher amounts of damage. A windy would do ~510 per hit. I know, it's not much, but couple that with Amplify Damage and you are at 1530 per hit. And this is of course assuming 50% PDR to start with.
I'm not saying that trappers are useless or something, they can be a good part of a team, but it almost requires 1v2 situations, which is not good. Imagine a trapper facing a Bonemancer and a BvC 1v2. How will the trapper have a chance of victory when the BvC deals such high damage as 1403 per Grief hit and 529 per Beast hit? Even if they had 4500 life, they would be toast in quite a short time, even more so considering the ~800 damage added to each two whirls from the Bone spears/spirits of a Bonemancer. The first two whirls that connect will alone deal 4135 damage, which I believe is enough to annihilate almost all Assassins.
That aside, a trapper has just as many ways to integrate with a team as a windy, giving ranged offense, locking ability as well as compatibility with necros and anything with conviction .
That's even more problems. You end up depending on Conviction and 1v2 situations. That means, at best, your team would look like BvC/Trapper/Bonemancer/V/T. Which does not look too good. Not that it would such either, just does not compare to some other teams.
Of course, in the end it all comes down to how the team duels. I, for example, would like a team where all the members can stand alone and wreck havoc as a team, ripping apart almost if not everything. I know, TvT's are supposed to be team-based duels, but there will always be a situation where one member of the opposing team will be facing one or more of the members of the opposing team with no other team members left. And if you ignore that fact, you are up for trouble.