One of my players REALLY wants to have the Ataru Striker specialization. he even went so far as to contact Sam Stewert to clarify the section on Page 194 "Becoming Force Sensitive"

In EotE, and AoR you need to take Force Senstive Exile or Emergent to become force sensitive. In F&D you get what is essentially 20XP for free and Force rating one

For me I think there should be a Universal tree in F&D before taking one of the careers. But I can understand why the rules on Page 194 exist for the ease of playing the game

They're not really getting 20 experience and a force rating for free. Characters that start in FaD only get 10 career skills (6 career + 4 bonus) and 5 free skill ranks in them (3 career + 2 bonus). EotE and AoR characters get 12 career skills (8 career + 4 bonus) and 6 free skill ranks (4 career + 2 bonus). In addition, a character starting in EotE and AoR may be spending 20 experience to get into Emergent/Exile, but they're still getting a second talent tree, and one that has both a Dedication and Force Rating talent in them.

As is, the ways it's set up more than just to prevent people from jumping into the "good stuff". It has narrative aspects to it. FaD characters represent people who have had a focus on the force for a decent chunk of their lives and in a way, it's what defines them. They've had a longer time or more experience growing with the Force, and putting more of their efforts into the Force. Somebody starting from AoR or EotE are characters that don't find out about their sensitivity until much later in their lives or it's more of just a secondary side part to them; they haven't had as much time to dedicate towards the Force.

Really, I'd suggest talking it over with the player. But really, it sounds more like he just wants to get into being able to use a lightsaber in his better characteristic for 30 less experience points (20 for Exile/Emergent, an extra 10 for taking Ataru which would be a third tree instead of a second) - which is really like 2 extra sessions of playing (and while you're free to, I wouldn't imagine you just tossing one to him as soon as he picked up Ataru anyways).

I've been playing in a few mixed groups and they work fine to me.

Lathrop has nailed it. Characters with careers from Force and Destiny do start out short-changed in skills, so they're not really getting getting a huge leap forward.

I also think Lathrop is right in that (s)he wants to play a character proficient with a lightsaber. There's nothing wrong with that, but you must be mindful that they will be expecting one at some point. That will probably be the biggest concern, but depending on your setting, the character will have to be careful with such a weapon.

Finally, Ataru striker does not have a Force Rating talent within the tree itself, so that may help mitigate your worry with them starting out with a Force Rating of 1 from within the career itself.

I assume this is a brand new game and this is a starting level character, otherwise if Seeker is not his or her career, then they will not benefit from any of the Force Rating talents until that character has picked up Exile or Emergent.

For me I think there should be a Universal tree in F&D before taking one of the careers. But I can understand why the rules on Page 194 exist for the ease of playing the game

I'm assuming this is an existing EotE or AoR character, in which case, how would that help? A F&D universal spec would still cost 20XP that he'd have to spend before taking the Striker. The EotE and AoR universal specs are perfectly fine trees with lots of useful talents suitable for an EotE or AoR background, why not pick one and then the Striker?