If you should side elspeth against both beatdown and control then shouldn't she be main decked?
In addition to what nikosison said, Paulo Vitor Damo da Rosa explained it quite well in this article:
”You side this card in every matchup, you should just maindeck it”
Nooooooooooooooo…. every time someone posts a sideboard guide and there is a card that gets boarded in a lot (and I mean every time), someone asks why it’s not in the main. There could be two main reasons:
1) The card is an answer to something that they are sideboarding. Take, for example, Phantasmal Image in UB – I boarded it in against almost everyone, but it was not in the maindeck because the reason I boarded it in was mainly to deal with Thrun, the Last Troll, and this card is usually not in their maindeck game one (or at least not in great quantities). As a matter of fact, I’d probably maindeck Image in UB nowadays, but that is because of the rise in Strangleroot Geist – if Ramp is the default RG deck, then you don’t want it main. If you’re playing a combo deck, then you might end up siding Echoing Truth against everyone, because you need an answer for their answer, but that doesn’t mean you need Echoing Truth main – you’re just not going to face any Leylines game one.
Sometimes, it’s not a specific card that they have, but a culmination of cards that makes one of your cards worse. Take, for instance, Goblin Guide and Steppe Lynx – those are two highly aggressive cards, good for racing but not that reliable in the long game. In game ones, that is good enough – they can’t deal with your cards so you just kill them. For games two and three, though, especially if you are on the draw, they will likely bring in answers and that will make the game a lot slower, at which point those cards are no longer very good and then you side them out for a card that is slower, but more resilient.
2) There is simply no room for the card. Sometimes, you need stuff in the main to deal with different decks and you can’t afford a generic card, but you end up having cards to side out against everyone, so you bring them in. To extrapolate – in a scenario where every deck is either Black or Red, you might maindeck four Circle of Protection: Black and four COP: Red. Then, in the board, you have four of a generic card – say, Loyal Cathar. If you play against a Red deck, you’ll take out COP:Black for Loyal Cathars; if you play against Black, you’ll take out COP:Red for Loyal Cathar. In every single match, you bring in four Loyal Cathars – but you still think it’s going to make for a better deck if you have those COPs main. For a more “real life” example, take a card like Nihil Spellbomb in UB – It’s in your sideboard for a reason, to deal with graveyard effects (say Frites). However, you have some dead cards against everyone – removal against control, Countermagic versus aggro – and you really don’t want those cards there. In this case, you might just take them out for Nihil Spellbomb, which draws you a card; this may lead to you siding Spellbomb in against almost everyone, but it definitely does not mean you want it in the maindeck.
Full article here
, if you're interested.