[The last thing the EU want or need is the UK taking part in EU elections (obviously different if A50 has been revoked).]
I've seen this said elsewhere but I'm not sure why? Is it a cost thing? I'd have thought it might be a good chance for Remainers to come out in force and actually elect some pro-EU MEPs, it feels like half the problem we've had is that our representation there is so poor so no positive messages return to the UK.
I think it relates to what I said upthread about the EU now seeing us a bad actors. (The person upthread who disagreed thought I was talking about the past, I think, rather than the current parlous situation mainly.) If we elect MEPs, we will be in a position to influence EU business going forward and in particular the EU budget. Considering the hostility of the current administration towards the EU, the extreme hostility to be expected from whoever replaces Theresa May, and the extreme and chaotic nature of British politics at present, I can see why this would be highly undesirable for them.
But I agree with you that, if possible, it could be a good opportunity for an electorate who are newly engaged in EU politics to make change, and if we managed to get rid of the likes of Farage it could send a positive signal to Europe that there is an appetite here for a different way forward. I am unclear why Westminster is so negative about electing MEPs, even the remainer MPs and softer Brexiters. They can always be recalled if/when we actually leave. The expense of holding EU elections is a drop in the ocean to the overall cost of Brexit and fucking Brexit up. And there are other options, such as leaving existing MEPs in their positions, or seconding MPs to Europe pro tem. As with Brexit generally, it does feel as though a whole spectrum of options is being closed down summarily for no very good reason, leaving us more and more boxed into an undesirable corner.