You have to look at this on a case-by-case basis. Generally regionalization and the localizing of decision-making is imo a good thing. But secession entails a change in living conditions that has to be taken into account. Once you start looking at what the consequences would be for the people who actually live in Texas, you'd immediately reject the option. An independent Texas would be impoverished, even more so once the bulk of the workforce decides to move to the US proper because they prefer living in a first world country. But in other situations (Scotland, Ireland, Wallonia, etc) the circumstances will be different.
Decentralisation is something I'm on board with in some instances, but secession is something I don't agree with in many cases at all. Of course every case is different, but those that are based on nationalist ideals or economic decisions are not cases I agree with whatsoever. I feel the same way about nations seeking to leave the EU really.
I mean, if we're going on a purely financial level then I'm from the most impoverished area of the United Kingdom - The Midlands. There's a 'Mercian independence movement', but in actuality the rest of the UK would benefit immensely from not having to subsidise the Midlands any more. Does that mean they should leave? Economic reasons for splitting up peoples are pretty abhorrent to me. Just like, as a European, I feel angry about the people who are protesting about Romania and Bulgaria's EU status due to financial reasons. In fact, I'm almost as angry at those guys as I am about those who're against it for RACIST reasons!
If we're speaking of situations involving legitimate oppression, with a ruling ethnic class discriminating against a populace, then sure, independence sounds a wonderful thing. The recognition of Palestine and its complete independence for example, are things I can get on board with. Same with the problems caused by Serbia-Montenegro. But these decisions weren't based entirely on economic situations at all. Hence why I believe that secession in the Western World is a vastly different kettle of fish.
Like TEXAS LEAVE BECAUSE HATE DEMOCRATICALLY ELECTED GOVERNMENT AND OBAMA WIN is weird. As are the Cornish and Mercian independence movements.