There are many champs that are significantly stronger with runs pages that are less common (shen, nasus, eve, and jungle fizz all have pages I don't use for other stuff really) and the extra pages can be huge.
However, if you don't want to run champs that have different scaling or run anything in a non standard role (i.e. Fizz jungle or top instead of mid) then that won't apply. What will apply is adaptability to to match ups. Even ignoring that with only an AD and an AP page you won't have a page for ADC or tank/devourer jungles, the difference between multiple AP or AD pages for top and mid is huge. For instance, usually as Heimer mid I want HP seals, but Zed can be a tough match up if he gets ahead and taking armor seals to reduce his poke plus exhaust to reduce all in makes the match up 10x easier. Having scaling armor/mr vs an easy lane opponent match up or flat armor/mr vs a hard early match up is a huge difference in mid game team fights and lane phase respectively. There are so many variations I could cover, but runes really help get that last bit of strength out of champs so if you aren't dominating your lanes they are probably a good investment. Plus they open up so many options for champs. If you are only playing champs when you have a good opportunity to make them strong then one AP and one AD page limits you to only champs that do well on those pages unless you are willing to play a champ with sub optimal set up.
A list of champs that have better options than standard AP or AD pages:
TF- CDR flat/scaling blues and MS quints possibly to increase ultimate up time, ability to roam, and kite potential. He wins through map pressure so CDR is a huge impact.
Shen-Passive scales with health and AS so having AD/AP rather than AS hurts his damage overall in lane role.
All ADC champs-AD is good early in lane but AS is a better stat for their scaling given the items they buy. AS quints with AD marks is better than an AD page.
Tank jungles- Sometimes mpen can be the best option (like amumu or maybe grag if you build partial damage build) for marks but usually AS marks are better to help with early clears, and personally I like MS quints on a lot of jungles to help pathing/ganking faster.
Yi/Xin/Jax/Noc/Teemo/Tryn/Any devourer jungle or champ that scales with AS through on hit or being primarily auto based for damage- They scale better with AS
Also while a basic AP page works pretty much any scaling AP mid wants to run scaling AP Blues if they don't need to take MR for the match up so having both options is good.
