"They are valuable illustrations of spiritual realities." So are the sacrifices.
That same argument could be used to justify sacrificing and eating a lamb at Passover, "all the while knowing it's an illustration of ..."
Is there a problem with that?
Of course not.
....except if a person is incapable of worshiping that way without it feeling to them like they are trying to be justified by what they are doing.
Saying that you don’t really mean your observance as an act of obedience but only as a symbolic gesture does not negate the fact that the very act of observing a commandment is an act of obedience, no matter your motives.
Just because you may be incapable of doing something for a reason other than a required obedience of law doesn't mean everyone else is incapable of that too.
I'll say it again.........take a big step of Christian maturity, folks........stop projecting your feelings and ways onto others. They aren't just like you. I know that it is the natural tendency of us all to think 'since I feel and think this way surely everyone else must to', but it's completely and utterly false. Everyone else doesn't think and feel the way you do. There are people who can keep a Mosaic observance without thinking they are trying to justify themselves and can do it out of love and appreciation for God. Especially when the alternative system of worship created for us by the early Catholic church is clearly pagan to them.
So the problem in observing the Mosaic Feasts, just like with observing the Mosaic sacrifices, is that you are observing the shadows and promises of what Messiah would one day do as if he has not yet come and accomplished everything those old shadows promised.
Just because you are incapable of observing the feast and sabbath cycle in appreciation and love for what God has done for us in Christ doesn't mean everybody else is incapable too. As I say, take a big step of Christian maturity. Let other believers be able to think different thoughts and have different feelings than you think they have to have because you have them. They belong to Christ too. If you are concerned that they are wrong God has accepted them too and is able to make them stand:
"God has accepted him. 4Who are you to judge the servant of another? To his own master he stands or falls; and he will stand, for the Lord is able to make him stand. "-Romans 14:3-4
That is the reason God removed everything He had provided for the Old Covenant's observance rendering it not only ineffectual but literally impossible to practice those things which in the former days made reconciliation and fellowship with God possible. Because in these last days he has made a new and living Way.