When an uninsured driver t-boned my X3, State Farm (my insurer) got me a crappy compact car for a month, and then left me carless for a month because my rental coverage ran out before the repair was complete.The problem with this is (assuming you are not at fault) that you enter a contractual relationship with your insurance company. If your policy provides for x days of rental, you may be stuck with your company's limitations.
In my experience (and I am not a plaintiff's attorney), one's own insurance company didn't want to provide the rental for the period necessary to find and obtain a like-replacement, even if the other party was at-fault. And one's insurance company will not be aggressive in pursuing diminished value.
In contrast, directly engaging the insurance of the at-fault driver gives you much more legal leverage if you know what to ask for to be made whole.
When a well-insured driver t-boned that same X3 a year later, State Farm got the other insurance company to agree to be on the hook for a like rental for the duration of the repair, so I got a midsize SUV for two months.