Slardar is a shit hero because he doesn't bring anything to a team
-25 minus armor is nice on paper so you can blow someone up fast but there are more than enough heroes to provide minus armor (and some of them do it an AoE or have to the potential to) that are actually useful for other things as well. The rest of his skills aren't any better.
How many games can you look back at and say "oh my god if they just had Slardar they would have totally won that game, all they needed is -25 armor!"
Negative armor counters agi carries that like to stack up on stat items (Gyro, SF, Morph, PL, etc) or naturally tanky heroes (BB, Centaur, Timber, etc).
To answer your question, yes there were games where I thought of how good it would be to have Slardar's ult against the enemy line-up, mainly because of heroes in the first group I listed before as they are the ones that are favored by the current meta.
Also regarding negative armor, consider that all the heroes that have it naturally aren't able to provide sure a massive debuff has Slardar does. You need something like VS+Naga+SF+BB to have
almost the same impact.
I remember notail saying that slardar can be played as a support
How does this work? You need blink to do anything and even after you get it you're left with a disable with a massive cast animation, so you'll never be more than a shittier version of Lion or SS.