I don't magnetise my models.
Many people do, nowadays. There are lots of tutorials on blogs and websites across the 'net showing you how to do it. There is one here on Mini Wargaming and a video here on Way of Saim-Hann. It's reasonably simple to do and doesn't cost much. In fact it saves you money in the long run.
The main reason for magnetising your model is to maximise your investment in the model; you can buy one Predator tank, for example, and fit it with heavy bolter sponsons, but then switch those out for lascannon sponsons without having to buy another tank. This also gives you greater tactical flexibility too, as you can tailor your army to your plan and opposition.
So it's easy to do, saves money and gives you a more competitive army. What's not to like?
The reason I don't magnetise my models is because I care about personality, theme and background.
I don't view models as just playing pieces in the game, but as part of a living breathing army in the 41st millennium. Those armies are defined by their weaknesses as much as their strengths.
A Predator tank in my Blood Angels army is not just a tank, it is Sacrum, a two century old vehicle which has fought in numerous campaigns. It is armed with an autocannon, heavy bolter sponsons and a pintle storm bolter. It earned it's name after it's first engagement against an Ork Waaagh. Leading from the front, it allowed several rhino troop transports to reach the Ork lines and despite being immobilised and drawing heavy fire, it spat a hail of death back into the greenskins. Without the sacrifice of that Predator, the Blood Angels may not have won the battle. Since then then, Sacrum has fought in countless battles, always utilising the same tactics, getting stuck in to the heart of the fighting and somehow surviving to fight another day.
Would it still be Sacrum if I swapped out the sponsons? I don't think so. It's the same with the weapons I choose for characters. Their weapons, wargear and armour become a defining part of them so I couldn't choose any others. It would seem like a betrayal!
I'd rather have the stones to pick the model as is, and try and make the tactics fit my force and the way my models are equipped. That might not be the 'best' choice for a game but at least you keep the background of your force pure and intact. I just think it's the start of a slippery slope switching between magnetised kit options. I don't want to start thinking of my models as just playing pieces. If I really want a certain combination I'll buy and paint a new model.
Besides, you never remember when your Predator with lascannons blows up an enemy vehicle because that's what you expect it to do, but you always remember when your Predator with heavy bolters does the same because the odds are against it. That would earn Sacrum a great honour, perhaps worthy of some extra painting on the model, a purity seal or maybe a write up in a battle report.
Anyway, it's often surprising how often I think I really need a particular combination of weapons only to find that I don't when I start playing the game. If after a few games it's clear I really do need a particular model or weapon then I build a specific model for that purpose. I will write up it's own history and then develop it further as I play some games with it.
I don't hate those who do magnetise their models, in fact I admire their ingenuity, but it's not for me.