For me it comes down to this... (the short version)
Basically it all comes down to, what is the easiest, most effective way to start to learn the game, and all it's in's and outs, and have the most fun doing it for the least amount of time and money.. and Bam, you have buy 3 IS mediums...
1, first and for most cost.. The fact is, you can outfit 3 IS mediums for just over 15m. that is slightly more expensive than one stormcrow with a few load outs, and cheaper than 1 heavy or assault clan mechs.
More often than not, people are not ready for a 45 million dollar C-bill grind to start playing, going IS, with a little beyond your cadet bonus, you have 3 unique mechs and get to start messing around.
2, which leads to variety, again the IS, buying 3 mechs for the cost you are going to have variety. Clan mechs, even different variants can play very similar with some slight changes. 3 good IS chasis either the HBK or shadow hawk, can run any system in the game, and also Boat weapons. You are getting every single play style available for every weapon system for peanuts compared to trying the same thing in three timbers, or SCR's.
3, Unlocking elites, To grind 45m in a basic'd mech, learning the game can be a rough ride. Elites make mechs feel totally different.. Personally i'd rather suggest a person see what it is like, before having to play 200 games, especially for people that can't grind all night, or weekend, But if you can grind 45-50m in a week, more power to you, I barely make 10m on a good week, and often that is with bonuses they add for events.. If i can pick up one module a week, i'm happy. 3-6m
4, Why as a new player would anyone suggest, putting a bullseye on someones head? It is just unkind to suggest that a newbie play one of the highest priority mechs on the field.
IS mediums don't suck!
5, While maybe not meta, the HBK and shadow hawks are both pretty deadly mechs. Good players play them with great success, and even in CW's. Are they the tops of the tops for competitive? Maybe not, at least not today, but who knows how things change. the top meta mechs people suggest 2 years ago, are different than the ones last year, 6 months ago, or even often 6 months down the road. (meta changes, period) But still, a player can do well in these mechs, and great players can do awesome in them. This game is far more about the player than the mech.
Learn on something not meta, then adapt to meta, not the other way around. Otherwise, all that leads to are the folks that whine and cry when their awesome most favorite mech gets nerfed.. It happens every single re-balance or balance patch. Play just under the meta, and you might get a buff too! You'd of been laughed at for giving the worst advice ever, try a dragon! (till quirks last nov, now you see um all the time) Like when quirks came out and they said it was Beta for quirks, and they will change.. people spent money, buying mechs, buying modules, and guess what the quirks changed.. which lead to many posts saying PGI sucks, i'm never giving you a single cent again after you nerfed my mech. (read posts from around dev 14th last winter and you will see what i am talking about.)
Lastly IS mechs, with swapping parts, especially XL engines, for people that like to play a very wide variety, for very little. I can buy a Light or medium chasis for around 5m, heavies a tiny bit more. But when sales hit, you can often pick up 3 whole chasis for just over the cost of a single module. Then just use your inventory to kit it out how i want. While someone is still grinding their first nova, i got 3 Spiders ready to rock and roll. You play more parts manager, but in the end it is cheaper, and a giant difference in feel from one mech to the next.
So often, do you buy an IS mech, strip it and build it how you wan't then use the engines, or even weapons in another mech. A lotta times it's more economical to buy a new mech for an engine and weapons, and save it for later especially with sales, sometimes you can buy a whole mech for the cost of its XL engine. Then you got something to play with when your bored. I've been playing for 3 years now, and i have a bunch of mechs i never even touched outside of a few drops, because i tend to collect mechs. But i cut myself off, no new mechs till i master every one of my 49 mechs! (i got 16 to go)
This game is about the long haul, not instant gratification. But nothing wrong with letting someone get hooked up in their own 3 mechs, and get some style to start that years long grind to a full bay..