It is not like you can avoid it unless you keep them hidden the whole time and only use them to finish off heavy damaged enemies. At that point I might as well not use one and have a more useful mech.
As soon as you move one into fire position, any alive enemies are not forced to attack your "tank" mech, they can attack any mech within range and why not the one that you can actually do some damage to.
The thing with the Spider is that it is the only 'mech other than the Assassin who can jump 7+ spaces. Jumping 7+ imposes a whopping +4 penalty to the enemy's attack roll vs the Spider (remember that we are dealing with 2d6 rolls here). Drop the Spider into dense terrain (+2 penalty) at medium range (+2 penalty again) and it is nigh untouchable. And 2 MedLas blasts to the rear (or the threat of 2) is no joking matter.
Again, this is far enhanced by Clan tech. In fact, two particular Clan 'mechs are notoriously well suited for this tactic: The Viper/Black Python and the Goshawk/Vapor Eagle. They have a combination of jump jets, large pulse lasers and targeting computer. Using the same tactic, they impose a massive penalty to you hitting them, but the pulse lasers and targeting computer mitigates a massive part of the penalties to them. Viper style 'mechs on TT are the most frustrating enemies you can face, especially when piloted by Clan warriors AND you don't have the Clan tech pulse lasers yourself. A Viper can easily go one-on-one vs a Daishi an come up on top. The only near contender is the Masakari variant with targeting computer and large pulse lasers, which has the armour to take the hits and enough bonus to hit to have a chance of swatting the bastards out of the sky.
That is all good for TT but I have not seen this dodge in the streams so far. All I have seen are Light mechs getting blown up as soon as they dare to try to contribute to the battle.
Base BTech rules are pretty straightforward: 4 + weapon range increment + enemy movement + your movement + terrain bonuses - your gunnery skill = number you have to roll to hit.
So, say a Locust runs at max speed up to your rookie pilot. Best case scenario is that the Locust ends up in the open right next to your rookie and your rookie didn't move that turn. That is a base of 4 + 0 (short range) + 4 (enemy movement) + 0 (your movement) + 0 (clear terrain + no intervening terrain) - 0 (rookie) = 8. That means a rookie recruit has to roll 8 or better on 2d6 to hit the Locust. Now, factor in your movement (1 for walking, 2 for running, 3 for jumping) and weapon range (2 for medium, 4 for long), and you very quickly find that you will miss far more often than not when you are trying to hit fast 'mechs with a rookie pilot.
THAT is the advantage of fast light 'mechs in TT. What is worse is the whole minimum range thing. Every hex within minimum range is +1 a to-hit penalty. A PPC has minimum range 3, that means a light 'mech that ran in to be right beside you (range 1) gives you a +3 penalty to-hit with a PPC straight off the bat. LRM has a minimum range of 6. Good luck hitting the light 'mech.
If the streamers are hitting left and right on light 'mechs, HBS has frakked up the whole game. Take it out back, Jim, and spread the ashes. End of story.