Indeed. WED-15, Mouse Droids, etc, belong on Huge ships, and I have no problem with an imperial Mouse Droid on the Raider (I'd love to see it (especially after the Robot Chicken version of the Mouse Droid!).
We have - see the other thread - a named imperial R2 astromech R2-Q5, so I'd be fine with that being a crew member who duplicated a generic R2 on an Imperial ship with a crew card. Putting it on Imperial snubfighters, though, is a no-no.
Firstly, in game terms, Imperial fighters are designed knowing they don't have access to them. The Defender's red turns, for example, are a big deal only because it can't take an R2 astromech. This is deliberate and shouldn't be removed 'just because'.
Secondly, astromechs don't and should never appear on Imperial Fighters because they just didn't (republic leftovers aside).
The Empire didn't want to pay for them, didn't need them, didn't want them and didn't like them.
Didn't want to pay for them - Astromechs are flexible and flexibility is expensive. A Rebel crew might find itself servicing and/or flying two or three fighter classes simultaneously but Imperial squadrons are purpose built, and - given the sheer number of TIE craft in service - you don't have to save much money to make a massive difference to someone's bottom line.
Didn't need them - astromechs are essentially a life support-light flight engineer, handling astrogation, power distribution between multiple systems, and similar duties. The general-production Imperial fighters don't get shields or hyperdrives, so don't have any use for them, whilst the 'elite' fighters get built with no expense spared and have in-built computers more than capable of handling such requirements.
Didn't want them - The other advantage of an astromech is a limited organic maintenance capability. This is neither required nor desirable for Imperial forces, who actively wanted to encourage fighters to be dependent on their base ships for maintenance and repair to make it harder to defect, and who considered fighters a disposable asset - such that any fighter damaged beyond combat effectiveness should just be written off.
Didn't like them - The Empire, as a whole, didn't like using droids for front-line combat duties prior to Palpatine's "Empire Reborn" (which had personnel limitations due to operating from the hidden bases around Byss). This is a seemingly universal thing, and probably stems from the fact that most of the Imperial admiralty and senior officers will have been commanders and junior officers during the Clone Wars. as a result, the phrase 'battle droid' probably provokes the same response in them that the phrase 'commie' provokes in US Military officers above a certain rank today.
If you want a 'navicomputer' modification which duplicates the effect of one astromech type, fair enough, but pick it carefully, don't just make it an open-ended slot. "This counts as that" just seems an inelegant approach.