Do I even need a middle tier

I am a developer here in the states with over 10 years of application development. I've worked with Mainframe 3270 Assembler, COBOL, C/VC++, VB, Java, and for the past 5 years .Net. AT my last job we had one web service for all of the ASP.Net apps to use for Security. I have done work with CORBA, RMI, and some other service calls / Remoting. I understand the n tier architecture.

My new job has several systems that utilize desk top applications, that in turn make request to BEA Tuxedo domains, which in turn retrieves data from an Oracle Database. One of these applications is written in VB6. Most of the other applications, due to acquisitions of other companies, are written in Java, C, Cold Fusion and numerous other languages. But I was hired (so I was told) to support this application and re-something (rewrite, re-arch, rebuild) to the application to bring up to date with newer technologies. I believe my supervisor is a little nervous because this will be their first .Net application and so there are a lot of eyes on its outcome.

Possible solutions:
I could rewrite the client portion of the application in .Net and still utilize Tuxedo for the middle tier.

I could remove the middle tier and make direct calls to the back end data using the FileNet Panagon API's.

I could write a lite client and utilize WCF with either IIS or a console service host application, which in turn makes the calls to FIleNet.

Finally, I could write a lite client and implement BizTalk as my middle tier.

Questions:
By having a middle tier multiple applications can utilize the same code. But wouldn't the middle tier add overhead and complexity because if a service changes all consumers are affected?

A service, by its disconnected nature, also adds to the process overhead, so would I want to limit their use?

I understand that if you have lot of dissimilar systems (IBM Mainframe, HP Unix, and a Windows 2003 Server), tools like Biztalk can tie those systems together to work more seamless. Other than that benefit, are there any advantages to using BizTalk over WCF services?

Is there a configuration I'm missing here?

What I would like to do is write a new application utilizing the existing FileNet Panagon / Oracle repository. The application would be written as a Intranet web ASP.Net solution. The client pages would utilize ASP Ajax and JavaScript for any client side processing while the server would utilize the Panagon API's and possibly calls to a security service (replacing LDAP with AD0).

This solution just seems too easy to think up and simplistic when compared to the system it is replacing (VB6 and Tuxedo). Yet it seems to me that solution would be a thinner, faster, and just as robust. I have one implementation of the application, on the web server. If too many users started using the tool, I guess I could add more web servers and somehow manage the load balancing.

Is the Tuxedo solution (middle tier) that much more robust than a two tier web app?

Any help you can offer with these questions would be greatly appreciated.