Here is the whole idea in
Orc-Pie-Room format. "To eat soup you need a spoon. But all you have is a knife. You want to eat the soup."
First you could get a spoon. Maybe you take a hammer to your knife, by which I mean you get new models for your existing army that
spoonify it. Maybe you go out and buy a spoon, by which I mean you go out and buy a spoon and trade it for models.
Second you carry on using the knife until you can get the job done. All the while looking over at the other guys spoon in abject disapproval. Toiling into the wee hours of the morning alone in a darkened room without so much as a picture of a picture of a picture of boobs, from the side, to keep you entertained
I see a lot of people dogmatically buying new spoon-like objects. "Well I could use more mobile firepower in my list to sort out Eldar so three Landspeeders. Better get those bought." Next week "hmm my Speeders are OK but really they need support so what about some Vanguard Vets with anti tank stuffs, that way when I play guard I can simply lay the pwn down. Get those." Now your problem is what is a spoon to Eldar soup is a Knife to Nid soup.
Really this is an article against min-maxing. The more focused your list is the less adaptable it probably is. What you want really is to arrive at the game and have your knife be blunt enough to eat soup with but sharp enough to still be a knife if you push the right way.
I am not suggesting you take a little from all boxes here. Pragmatically I assign primary and other role to all my units. XV88's. Primary role is blowing up tanks. Secondary role is moving up and using they're cheap plasma to melt attacking element face. Another secondary role might be absorbing non-power weapon low strength high attack mobs in close combat. Compare this to a Hammerhead. Primary role is blowing up tanks. Secondary role is providing mobile cover to stuff and being shot at. I know which I prefer.
When your opponent presents a certain kind of soup, no vehicle soup for instance, you just switch to secondary. I mean when was the last time you used your strength 7 Plasma Gun to shoot a rhino? Well sometimes you should be because sometimes that is the best path.
By all means go out and buy cool stuff but don't expect it to solve all your problems - just one or two narrow ones. At the same time if you use the same stuff in the same way your life is going to be dull and repetitive. Get yourself a knife that is just sharp enough and is a serviceable spoon to boot and I promise you will have fun and win more games.